With Vatican permission, we made acoustical measurements in St. Peter's Basilica in May 1970. The reverberation times for this largest church in the world (20 000 000 ft3) are much shorter than expected; those for the empty Basilica at midfrequencies being about 7.5 sec. This unexpectedly low value results because St. Peter's is a group of five large churches and a number of smaller spaces weakly coupled acoustically. Analysis by standard reverberation theory and simple considerations of acoustical coupling between the spaces agrees with the observations. Measurements made in 1966 with a large congregation gave reverberation times of about 5 sec. In 1964, when the Basilica was filled with highly absorbing seating for the Ecumenical Congress, the reverberation times were about312 sec. For the 200th anniversary Beethoven celebration on May 23, 1970, a complete concert‐hall stage was installed in the rotunda and the acoustics for Miss Solemnis were judged excellent by critical musicians. The exceptional acoustical properties are due to the multiple coupled air spaces into which the Basilica is divided, and to the diffusion produced by the wealth of architectural detail and sculptural ornament.