The life cycles and reproductive traits of Idotea baltica and I. chelipes in the Limfjord, Denmark were studied. The species had two annual generations of breeding females. However, while the first generation of I. baltica produced one or two broods that of I. chelipes had two or three. The second generations were only ovigerous once. The number and size of stage 1 eggs of both species were proportional to the length of the parent female. The relationships changed during the breeding period, and in early summer the overwintered females carried many small eggs compared to the few and large ones of the later, second generation. The reproductive effort of I. baltica females was greatest in early summer, whereas it remained constant in the case of I. chelipes. The breeding biology of the two species from other geographical areas is compared with the present findings.