We describe the life history of harbor porpoises in the Gulf of Maine by examining 239 animals killed in gill net fisheries and comparing these findings with the results of previous studies from the Bay of Fundy. Most female porpoises matured at age three and became pregnant each year thereafter. Reproduction was strictly seasonal, with ovulation, conception, and parturition occurring in the spring and early summer. The oldest specimen in the sample was 17 yr of age, but most individuals were younger than 12. The findings are similar to those of earlier studies from the Bay of Fundy and support the hypothesis that these animals form a single population. Harbor porpoises represent one end of a continuum of odontocete life histories that spans a wide diversity of strategies. In comparison with other, large odontocetes, harbor porpoises mature at an earlier age, reproduce more frequently, and live for shorter periods.