The influence of elevated water temperatures on fecundity ofCaenissp. emerging from large outdoor channels was investigated during the summer of 1980. Four temperature levels were provided: Ambient (Tennessee River, U.S.A.), and approximately 3°, 6°, and 9° C above ambient. Number of eggs per female ranged between 212 and 1787 at all temperature levels. A median test indicated a significantly lower number of eggs in females from +9 channels than from the other temperature levels.