A cooperative study based on an agreement between the National Marine Fisheries Service and the Makah Indian Tribe was conducted during 1988-90 to assess the nature and magnitude of harbor porpoise (Phocoena phocoena} interactions in the Makah chinook salmon set-net fishery. The Makah set-net fishery operates annually along the northern coast of Washington State (USA) in the North Pacific Ocean and in the western Strait of Juan de Fuca from 1 May to 15 September. The fishery targets on chinook salmon (Oncorhynchus tshawytscha) using submerged gillnets up to 100 fathoms (200m) long. An observer program was conducted during the 1988-90 seasons and fishing effort was estimated. A total of 138 harbor porpoises was observed or reported taken incidentally during the three years of which 100 were collected and necropsied. Harbor porpoises were primarily taken during a one month period from mid-July to mid-August at the Spike Rock fishing grounds in the Pacific Ocean. The number of harbor porpoises observed or reported taken in the fishery declined dramatically during 1989 and 1990 due to low fishing effort. Of the harbor porpoises collected, 55 were males, 45 were females, 100 were aged and the reproductive condition of 99 was determined. The maximum estimated age (based on growth layer groups within the dentine) was five for females and eight for males. A large proportion (54%) of the aged porpoises were one and two years old. Most (63.6%) of the 99 animals examined were reproductively immature. Males were reproductively mature at age four with a body length of approximately 132cm. Females were reproductively mature at age three with a body length of approximately 155cm. The principal prey of both harbor porpoise and chinook salmon were Pacific herring (Clupea harengus pallasi), market squid (Loligo opalescens) and smelt (Family Osmeridae).