The cladoceran Bosmina Longirostris undergoes a peculiar egg—size cycle in Frains Lake. Females carry small eggs during summer, then switch to large eggs in late fall, in the process more than doubling the enclosed yolk volume. Winter generations mature at larger sizes, then shift to small eggs in early spring. The fluctuations in egg size are not strongly associated with indirect measures of nutritional conditions. Rather, they seem part of a cycle that produces small young in one seasons, large young in another, primarily as a response to changing patterns of predation.