Partial migration, the behavior pattern in which a portion of a population migrates while others do not, is a widespread phenomenon with ecological and evolutionary consequences. Most coho salmon Oncorhynchus kisutch from Puget Sound, Washington, migrate to feed over the continental shelf or offshore in the North Pacific Ocean, but some remain in the semi-estuarine waters of Puget Sound and are termed residents. The objective of this study was to determine which of several factors influenced residency in Puget Sound coho salmon. Coded–wire tag recovery data showed that resident coho salmon were smaller than their migratory counterparts, and we used this size difference and the relative catch patterns along the coast and in Puget Sound to classify coho salmon caught in Puget Sound between November and August as residents. We then analyzed the effects of location of origin, day of release, weight at release, hatchery or wild rearing, and year on the proportion of fish caught as residents. Based on 268 release groups between 1975 and 1992, we classified 3.4% of fish recovered as residents, 61.3% as migrants, and 35.3% as ambiguous because they were recovered in Puget Sound in September and October, when residents and migrants were mixed. The proportion of residents varied as a function of year, basin, and day of the year. Releases into south Puget Sound produced the highest proportion of residents, and resident fish tended to be recovered in the basin where they entered Puget Sound. While other factors may influence residency in coho salmon, the effects of day of release and location of origin may be useful for management of these populations, as the tendency to remain in Puget Sound or migrate to the coast affects the fisheries in which the fish are taken and their growth rate, their uptake of contaminants, and their role in food webs.