The reproductive condition and body mass changes of Idotea baltica were studied by repeated sampling during the maturation and breeding period in 1986. Our observations indicate that I. baltica females have a capacity to produce more than one brood during warm summers in the Finnish SW archipelago of the Baltic Sea. First, new ovarioles may develop in gravid females while they are still carrying the late embryonal stages of their earlier brood. Second, there are two successive peaks of gravid females in June andJuly, and furthermore, the initiation of these peaks coincide with a high frequency of gravid females with broods at early developmental stages. Third, the condition index of gravid females increases during the incubation period and is extremely high in post-gravid females. This is in accordance with the earlier observations about the increase of food consumption and activity of females just before or after the release of juveniles, and would be a reasonable tactic for an iteroparous breeder.