The cochito (Phocoena sinus), the smallest member of its genus, is known from 21 confirmed records from the upper Gulf of California and is probably found only in that area. Diagnostic and general morphological characters are given, including the external measurements of two specimens; little to no data are available on other aspects of its biology. Affinities of the axial skeleton and external morphometry suggest that P. sinus evolved from a Pleistocene stock of Burmeister’s porpoise (P. spinipinnis). P. sinus has been taken incidentally the gillnet fishery for totoaba, Cynoscion macdonaldi, perhaps since the late forties, and the annual kill during the early seventies may have been in the tens to hundreds. Although the fishery for totoaba was banned in 1975, a small number of cochito will probably continue to be taken in other fisheries and this may have considerable impact on the small population. Research is needed to determine, among other things, the life history of the species and the effects on it of incidental capture in trawl and gillnet fisheries.