We examined the nature, magnitude, and impact of incidental catches of harbor porpoises (Phocoena phocoena) by groundfish gill nets in the southwestern Bay of Fundy [Canada, USA] between June and September 1986. We estimated that the total catch for the year was 105 .+-. 10.8 animals (SE). Nets in areas with a high density of porpoises caught more animals than those in other areas. Apparently, porpoises are entangled while nets are on the bottom in water depths of 37-96 m. We did not detect any change in porpoise density between 1981-1986, but 2 significant changes in length frequencies (an increase in length of calves and an absence of large porpoises in the recent sample) have occurred since 1973. These changes may be attributed to the gillnet fishery, which has been operating for 10-15 years. Given the slow reproductive rate of harbor porpoises, these incidental catches seriously threaten the population.