Still a ‘grey zone’ of the history of Islam in Central Eurasia, the period from wwii to Perestroika in the Volga-Ural region of Russia is dealt with in the present study through a rich documentary material from the National Archive of the Republic of Tatarstan. The author notably casts light on the collusions between the Muftiate and local non-registered communities of faithful, and on the pressures (fiscal, among others) exerted, not always successfully, by the Soviet authorities for putting an end to this state of affairs. A chronological reconstruction of the measures implemented in the Tatar ASSR permits the author to divide the period into different phases: (1) the opening of sixteen registered mosques in the Tatar ASSR between May 1945 and December 1947, and the parallel functioning of at least twenty-five non-registered mosques in the years following the wwii; (2) the slowing down of measures favourable to local communities of faithful as soon as 1948, followed by the closure of two rural mosques in the mid-1950s, and by the brutal turn of 1958 resulting in the limitation of registered mosques in the autonomous republic to the number of eleven; (3) new measures taken in 1961 by the Council of Ministers of the USSR against religious practice, and their echo in the Tatar ASSR—local authorities identifying, that same year, the existence of 646 non-official mosques served by 366 illegal mullahs!—; (4) a gradual liberalisation in the late Brezhnev period, permitting the registration of five new mosques (and two Orthodox churches) in the Tatar ASSR between 1976 and 1981. In all, this short article brings a significant contribution to our knowledge of the implementation of the Soviet religious policy in a religiously active national republic of the RSFSR, the author insisting at the same time on the silence of the official sources on which he has been relying as for the real dimensions, quantitative and qualitative, of underground religious practice. From this viewpoint, this article bears interesting testimony of the resources and limitation of classical historical approaches as far as Soviet Islam is concerned. Unfortunately, as it is almost always the case in publications emanating from the former USSR, the varied documents used by the author have not been described.