SUMMARYAnalysis of the seasonal occurrence of Argulus coregoni on Oncorhynchus masou and Salmo gairdneri indicates that the parasites overwinter as eggs and that there are 1 or 2 generations/year. Larvae which hatch from the overwintered eggs are abundant from May through July. Individuals hatched in May and June attain maturity and lay eggs in August. Larvae of the second generation are abundant in September, and they lay eggs in October–November which overwinter. Hatched larvae from overwintered eggs may mature in July and deposit overwintering eggs in September. The sex ratio of A. coregoni is about 1:1. A. coregoni shows an age-specific site preference on O. masou. Small parasites occur on almost all parts of the skin and fins, with slightly heavier concentrations on and around the pectoral and pelvic fins. Large parasites are found mainly on the skin around these fins.

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