We present the results of an aerial line transect census of cetaceans along the California coast during March and April 1991. This survey was the first of two aerial surveys designed to estimate cetacean abundance within approximately 100-150 n.miles (185-278km) of the coast. A primary team of two observers searched through bubble windows which allowed an unobstructed view of the trackline directly underneath the aircraft. A third, semi-independent observer searched through a belly window and made observations of animals that were missed by the primary team. A total of 155 sightings were made of a minimum of 14 cetacean species (some animals could only be identified to higher taxa). Of those groups that were on the trackline, the fraction seen by at least one team, g(0), was estimated to be at least 0.821 for groups of ≤10 small cetaceans, 0.956 for groups of >10 small cetaceans and 0.977 for medium and large cetaceans. Estimates of abundance using these correction factors are 277,783 common dolphins (CV = 0.46), 10,506 Risso’s dolphins (CV=0.48), 46,334 Pacific white-sided dolphins (CV=0.78), 13,362 northern right whale dolphins (CV=0.41), 1,236 harbor porpoise (CV=0.45), 5,832 Dall’s porpoise (CV=0.28), 4,011 bottlenose dolphins (CV=0.62), 117 minke whales (CV=0.68), 1,720 gray whales (CV=0.42), 405 humpback whales (CV=0.42), 87 killer whales (CV= 0.76), 56 sperm whales (CV=1.07), 505 beaked whales (CV=0.44) and 59 fin/sei/Bryde’s whales (CV=1.00).