An exogenous strain of cultured Hexamita salmonis (Moore) was employed to induce trophic hexamitiasis in otherwise disease‐free juveniles of coho salmon (Oncorhynchus kisutch) and steelhead trout (Salmo gairdneri). Mortality and growth were the parameters used to detect the effects of hexamitiasis on the two species. Two levels of each of the three experimental factors under study, Hexamita infection, species of fish, and density of fish, were arranged in a three‐way factorial design. Replicate lots involved a total of 1,440 fish held under controlled laboratory conditions. Comparisons of growth and mortality indicate that infection with H. salmonis over a period of 8 weeks is innocuous to coho salmon. Steelhead trout suffered a low, but statistically significant mortality which subsided after the sixth week; growth rate was not affected.

Trophozoites of Hexamita salmonis, asserted pathogen of juvenile salmonid fishes, were isolated from two species of Pacific salmon hosts and cultured repeatedly in an organic medium saturated with nitrogen. Primary isolates and serial subcultures usually exhibited five- to tenfold population increases per passage.

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