The life cycles of the three most frequent amphipod species of the shallow waters of Kiel Bay (western Baltic Sea) were studied by evaluating their length-frequency distributions and percentages of gravid females over a15-month period. B. sarsi has two generations per year. The first one hatches in late spring and reproduces insummer. Its offspring overwinters and closes the cycle by breeding in spring. The reproductive phase in the population lasts from May to November; the minimum temperature for reproduction is about 6 C. M. gryllotalpa has a life cycle similar to that of B. sarsi, but less pronounced breeding periods and a shorter reproductive phase (end of June to October). C. insidiosum produces three generations per year during theperiod from end of April to beginning of November. Reproduction culminates in late spring/mid-summer, and to a lesser degree, in autumn. In the last two species variation in sex ratio shows a relationship to thereproductive cycle.