In late November Mycielski goes to the Soviet Union for the first time. During his two-week trip he visits Moscow, Leningrad and Erevan. He includes a detailed account of the visit in his diary.

Mycielski takes part in the preparations for the opening of the rebuilt Teatr Wielki-Polish National Opera (19 November). In protest against the politicisation of the work of the Programme Board in February 1966 Mycielski hands in his resignation. 

He composes the first three songs of the cycle Krągły rok  [All Year Round] to words by Jarosław Iwaszkiewicz (he will complete the cycle in 1968).

Mycielski takes part in the preparations for the celebrations of the millennium of Poland’s statehood and Christianity. Disgusted with the actions of the government, he writes to Cardinal Wyszyński, Primate of Poland:

The celebrations of the Millennium of the Baptism of Poland, which the Church, together with the entire Polish nation, wanted to be a manifestation on an international scale – by inviting the head of the Church and representatives of Episcopates from various countries – are unfortunately boycotted and sabotaged by the authorities of the Polish People’s Republic. [...] It is difficult to guess what the authorities, who call themselves guardians of public order, are aiming at. I am convinced that they are acting in a way which does not give them the support that any authority should have in a free society. [...] I cannot speak publicly to express these thoughts and these feelings. In our country the Press Control Office not only does not allow the opposition, which could control those in power, to speak, but it does not even allow critical opinions or even just contributions to discussions to be published. (27 June 1966, "Dziennik 1960-1969", Warsaw 2001)

To celebrate Nadia Boulanger’s eightieth birthday Mycielski composes Five Preludes for string quartet and piano. In addition, he completes Symphony No. 3.

Second trip to the USSR. 

On 29 February, during an extraordinary meeting of the Polish Writers’ Union, a letter by Mycielski is read, expressing his protest against censorship (blocking of the production of Adam Mickiewicz’s Forefathers’ Eve directed by Kazimierz Dejmek).

On 10 April Mycielski is dismissed as editor-in-chief of Ruch Muzyczny.

He fails to get a permit to travel to Monaco.

On 1 September he writes a “Letter to Czech and Slovak musicians” expressing his protest against the attack by the Warsaw Pact troops on Czechoslovakia. The letter is published in a special October issue of the Paris Kultura.

As a result, the authorities impose sanctions on Mycielski – his works are banned from publication and performance. 

The composer receives the Alfred Jurzykowski Prize from the Jurzykowski Foundation in New York.

Once gain Mycielski is refused passport to travel to Monaco.

The increasingly severe sanctions make it impossible for him to earn a living. Desperate because of his worsening situation, he sends a letter to the Prime Minister, Józef Cyrankiewicz. He writes in it: 

I have never been punished by a court in Poland or deprived of my civil rights. For nearly two years I have been discriminated against without being informed on what legal basis this is going on.  [...] I ask you, Prime Minister, to examine these directives or groundless initiatives, which make it impossible for me to work and live in this country, of which I am a loyal citizen. (3 November 1969, quoted in "Niby-dziennik", Warsaw 1998, entry of 3 July 1970) 

After a break of two years Mycielski receives his passport and is allowed to travel to France and Monaco. He also gradually returns to the pages of Ruch Muzyczny.

Mycielski’s Symphony No. 3 is performed at the Poznań Music Spring Festival.

In September the symphony is also presented at the Warsaw Autumn.

Mycielski receives a regular column at Ruch Muzyczny, "Twice a month" (later called “Once every two weeks”).

Together with Zbigniew Herbert he prepares and signs the so-called "Letter of the 15" demanding that Poles in the USSR be granted access to Polish culture and education.

Towards the end of February Mycielski goes to South Africa to visit his brother Kazimierz. He writes to Andrzej Panufnik:

[...] I see my brother, sister-in-law, the house with numerous family mementoes, dancing black men with feathers on their heads, arms and legs, tomorrow we’re going on a trip to the Indian Ocean, to the Zulus, I hope neither they, nor the sharks or rhinos will eat us. Here, on a plateau of 2000 m., the landscape and climate are seemingly European, but huge, wide, in addition, the sun goes from east to north and the south is in the shade and one walks upside down, but I have not fallen yet, head down, on the Southern Cross. (26 February 1975).

On 11 April, at the Pallottine Fathers’ Paris Centre of Dialogue, Mycielski gives a lecture on his music. 

He signs the Memorial 59 expressing protest against the planned changes in communist Poland’s Constitution.

Mycielski’s Symphony No. 4 is performed at the Poznań Music Spring Festival. After the premiere Ludwik Erhardt writes:

Zygmunt Mycielski’s Symphony No. 4, completed in 1973, is music that is very beautiful and perhaps too concise because of that ("Ruch Muzyczny" 1976 no. 11)

Celebrations of the 50 years of the Association of Young Polish Musicians in Paris. Mycielski takes part in meetings devoted to the event; his Five Wedding Songs are performed during one of the jubilee concerts.

Polskie Wydawnictwo Muzyczne publishes the third volume of Mycielski’s writings, Postludia [Postludes]. 

The composer completes Symphony No. 5. Mycielski celebrates his 70th birthday during the Musical Encounters in Baranów Sandomierski.

First trip to California. At Mima Porter’s ranch Mycielski works on Six Songs for Orchestra.

His Symphony No. 5 is performed at the Poznań Music Spring Festival.

Mycielski begins collaborating with the underground magazine Zapis. Asked by Adam Michnik, he publishes there excerpts from his Diaries, and writes about the revival of the hitherto proscribed music of Andrzej Panufnik (in the context of the performance of Panufnik's Universal Prayer at the 1977 Warsaw Autumn). 

Second visit to California. Mycielski completes there his Variations for small string orchestra.

His Symphony No. 4 is performed at the Warsaw Autumn.

Mycielski receives the Award of the Minister of Culture and Art.

He publishes two “Letters from Poland” in the Paris Kultura.

He goes to California for the third time and composes Fantasia for orchestra. Before returning to Poland, he writes in his diary:

These last few days of travelling here, from where I am about to board the Swissair Zurich-Warsaw flight, have been like jumping off a high trampoline into cold water. This cramped room of mine, these parcels, this helpless and hopeless life there...! (9 August 1981, "The Last Quasi-Diary" 1980-1987, Warsaw 2012) 

Martial law is in force in Poland. Mycielski fears for Adam Michnik and other members of the opposition.

He focuses on his creative work and completes Three Psalms.

Mycielski publishes a letter signed “Zyndram Wiśniowski” in the Paris Kultura. Considering Poland’s situation, he writes in it: 

To remain still? To wait for the world’s biggest empire to collapse? Or to rebel from time to time in the hope that it will become like Western Europe, restoring to Poland its status of bulwark of Christianity and a civilisation to which we belong and have wanted to belong for ten centuries? ("Kultura" 1983 no. 7-8).

Mycielski’s fourth visit to California. He composes there Eight Songs to Words by Zbigniew Herbert for baritone and piano.

In September Psalm XII is performed at the Warsaw Autumn.

Last visit to Ojai. California is consumed by wildfires, Mycielski and his host have to be evacuated. 

He is working on Symphony No. 6, which he will call Last Symphony.