Persistent organochlorines (PCBs and p,p′-DDE) in the bodies of dalli- and truei-type Dall’s porpoises (Phocoenoides dalli) from the Bering Sea and the northwestern North Pacific were used as chemical tracers in evaluating some reproductive parameters of this species. In females the concentrations of organochlorines decreased consistently with age after 2 years. The results of isomer-specific analysis indicate that the lesser chlorinated biphenyls were preferentially eliminated from adult females. Elimination probably occurs mainly via lactation rather than through the placenta during gestation, since changes in congener patterns in female porpoises follow closely the parturitional history of the female animals. The organochlorine data indicate that females of Dall’s porpoise attain their sexual maturity at about 2 years of age and their first parturition occurs around 3 years. The active parturition period seems to be up to approximately 6 to 7 years of age and declines thereafter.