Coho salmon (Oncorhynchus kisutch) and rainbow trout (Oncorhynchus mykiss) are commercially cultured in net-pens in Onmae Bay, Onagawa, Miyagi Prefecture, Japan, for eight months from November to the following July. Eight monthly samples of five rainbow trout and three bimonthly samples of five coho salmon were taken from two adjacent net-pens in this bay to study the infestation patterns of salmon louse (Lepeophtheirus salmonis). The results indicate that (1) rainbow trout was highly susceptible to salmon louse, (2) coho salmon harbored only the adult and preadult stages of the salmon louse, (3) the parasite reproduced twice on rainbow trout during the study period, and (4) the prevalence and mean intensity of infestation increased steadily into the harvest time (in July) on both species of salmonids. It is concluded that, aside from the resistance of coho salmon to infestation by salmon louse, the farmers' practice of rearing only the young fish and harvesting the fish in less than a year of culture accounts for the mitigation of salmon louse problem in Japan.