This paper reviews available information on the population biology and incidental mortality of the vaquita, Phocoena sinus. A re-examination of previous records and the collection of new records reveals that this porpoise has the most limited distribution of any marine cetacean, being restricted to the uppermost Gulf of California, Mexico. Although no reliable abundance estimates are available, the population is very small, perhaps only in the low hundreds. Little is known of the life history of the vaquita. Twelve neonates examined ranged from 67cm to 78.2cm in length and a near-term foetus measured 71.5cm; parturition occurs in spring, with a peak in late March-early April. Sexually mature females ranged from 135 to 148.2cm in length and sexually mature males from 128.3 to 144cm. The largest immature female and male were 128.7cm and 127cm in length, respectively, and the largest adults were a 150cm female and a 145cm male. Vaquitas are taken incidentally in gillnets, especially those nets with mesh sizes of 15-30.5cm. There are records of 128 vaquitas captured incidentally between early March 1985 and early February 1992: 65% in illegal and experimental gillnets set for a sciaenid, the endangered `totoaba’ (Totoaba macdonaldi); 28% in gillnets for sharks and rays; and 7% in gillnets for mackerels (Scomberomorus spp.) and in shrimp (Penaeus spp.) trawls. These 128 captures certainly represent only a fraction of the total mortality from fishing operations. At least 35 vaquitas are killed each year by these industries. Considering the probable low population size, the relatively high rate of mortality in fishing operations and the difficulties and the costs of implementing and enforcing long-term conservation measures immediately, I conclude the vaquita is in immediate danger of extinction.