Juvenile rainbow trout (Salmo gairdneri) were exposed to infection with Ceratomyxa shasta transferred in groups of 25 to aquaria, and tempered to various temperature levels increasing from 3.9 to 23.3 C (39 to 74 F) by 2.8 degree C (5 degree F) increments. During a 237-day observation period mortality in the experimental groups averaged about 80%, and was independent of water temperature between 6.3 and 23.3 C (44 and 74 F). No deaths occurred at 3.9 C (39 F). The geometric-mean time from exposure to death, however, was definitely a function of temperature, increasing from 14 days at 23.3 C (74 F) to 155 days at 6.7 C (44 F).A similar experiment with juvenile coho salmon (Oncorhynchus kisutch) gave quite different results. In this species, percent mortality due to C. shasta increased progressively from 2% at 9.4 C (49 F) to 22% at 15.0 C (59 F) and 84% at 20.5 C (69 F). No deaths occurred at 3.9 or 6.7 C (39 or 44 F) during the 201-day experimental period. As with the rainbow trout, the geometric-mean time from exposure to death was temperature dependent, increasing from 12.5 days at 23.3 C (74 F) to 146 days at 9.4 C (49 F). The data indicate that ceratomyxosis in coho can be suppressed by water temperatures of 6.7 C (44 F) or below.