Samples of blubber from 60 male and 55 female harbour porpoises (Phocoena phocoena) collected from 1969 to 1973, all but 11 from the east and west coast approaches to the Bay of Fundy, were analysed for DDT residues; the results were then compared according to sex, age class and year of collection. Factors complicating the study included variation of the levels of residue in individuals of the same age, perhaps due in part to differences in diet, and the relatively small size of samples. DDT residue levels showed a positive correlation with age in male harbour porpoises and a negative though less pronounced correlation in females, indicating that a significant portion of the accumulated DDT may be passed by females to the foetus or to the calf during lactation. It appears that DDT levels in porpoises of both sexes collected at the end of the study were very markedly lower than those of animals collected at the beginning. No close correlation of residue levels with body size was apparent nor was it possible to detect whether there is any significant annual fluctuation in residue levels. Differences between residue levels in some of the animals collected in North American waters outside the Bay of Fundy and the average levels of the Bay of Fundy group, may be due to differences in reproductive history, in diet, in oceanic conditions and in the amount of DDT that has entered the different habitats.