Burmeister’s porpoise (Phocoena spinipinnis), one of the most common small cetaceans in Peruvian waters, suffers the second highest fishing mortality. Biological data obtained from porpoises caught in gillnets off Peru from 1983 to 1989 are presented. The largest specimens are a 183cm female and a 182cm male (n=402), although on average, mature males are significantly (p<0.0001) larger than females: 170.3cm (n=63) versus 165.5cm (n=56). The relationship between standard length (X) and body mass (Y) is best described by the equation log Y = -0.714 + 2.48 log X (r=0.97; n=123; p<0.0001). Sexual dimorphism is statistically significant (p<0.05 and p<0.005) in eight cranial measurements and one meristic, but not in the condylobasal length. The correlation between condylobasal length and body length is relatively low (r=0.72). Sexual dimorphism in the colour pattern of the ventral field is described. A characteristic of this species is the bilateral asymmetry in the shape and extent of the flipper stripe. Minimum and average length (50%) at sexual maturity are respectively 153cm and 154.8cm in females (n=56) and 158cm and 159.9cm in males (n=63). The pregnancy rate is estimated at 0.60. Of the pregnant females, 34% were lactating. No evidence for a male seasonal reproductive cycle was found but the sample size was small (n=37). Periodicity in occurrence and size of foetuses and neonates suggests a gestation period of 11-12 months, with a peak of conceptions in summer. The sex ratio of foetuses and small neonates did not deviate significantly from 1:1 (chi2, p >0.50). Length at birth is estimated as approximately 86cm. The Burmeister’s porpoise mainly feeds on fish, including Engraulis ringens, Sciaena deliciosa, Anchoa sp. and Merluccius gayi. Endoparasites found include the trematodes Nasitrema globicephalae, Synthesium tursionis, Pholeter gastrophylus and Braunina cordiformis, as well as the nematodes Anisakis typica, Stenurus sp., Halocercus sp. and Pseudalius inflexus. Ectoparasites recorded are Isocyamus sp. and Xenobalanus globicipitis. Paita Bay (05″s) is confirmed as the northern distribution limit of the species on the Pacific coast of South America.