In a chalk stream in southern England Pisidium amnicum (Müller) released one annual brood of young in May. A small proportion of the largest adults, surviving brood release, initiated a second brood during August. In the present study these adults died before the young were fully developed. It is suggested that an extended life cycle may occur in other habitats. A slow-growing embryo brood developed in a steadily increasing proportion of the adults from August to April. The number of embryo shells was related to adult shell length. Mean brood size was 13 individuals, with a maximum of 37 recorded from adults of 80 and 85 mm.