Abstract
In an attempt to determine the seasonal fluctuation in the population of the stomach trematodes Hemiurus communis and Derogenes varicus of the cod (Gadus callarias), ten fish were examined monthly from May 1965 to May 1966. It is shown that the number of parasites increases with the size of the host, and the seasonal fluctuation is more evident in larger fish than in smaller ones, although the pattern is the same. The parasite load is mainly due to H. communis which has a fluctuation pattern much the same as that of D. varicus. The seasonal variation in the trema-tode population seems to agree with the fluctuation in the population of Acartia.

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