Low‐frequency vocalizations were recorded from fin whales,Balaenoptera physalus, in the Gulf of California, Mexico (mainly the Sonoran coast) during three cruises in March 1985, and February and August 1987. The predominant sounds were patterned 20‐Hz pulses. In March 1985, they were in sequences characterized by regular interpulse intervals of 9 s, whereas in August 1987, they were mainly in sequences of doublets in alternating 5‐ and 17‐s interpulse intervals. In February 1987, no patterned pulse sequences were detected. The 20‐Hz pulses were about 0.5‐s duration, typically sweeping from 42 to 20 Hz. Other fin whale vocalizations were of similar duration and typically modulated downward in frequency, averaging 82, 56, and 68 Hz, respectively, for the three cruises. Gulf pulses differed from those elsewhere in terms of frequency sweep, sound level, and time interval patterns. This uniqueness points out the possibility that fin whales in the Gulf of California may represent a regional stock based on their sound characteristics.