An experimental study of the copepod Mesocyclops edax documented that females which reproduced throughout their lifetime survived at a lower rate than unmated females which did not reproduce. This cost of reproduction was most apparent in animals receiving periodic, non-lethal temperature stress. However, among reproductive females, no significant correlations were detected between reproductive output and survival, suggesting caution concerning the application of cost of reproduction arguments to a population of females in which clutch size is seen to vary.