The cumacean Cumopsis goodsir (Crustacea: Peracarida) was sampled monthly in 1991–1992 in the surf zone of Creixell Beach (north-western Mediterranean), to describe its life history and secondary production. In this unstable coastal soft-bottom area, C. goodsir is one of the dominant species of the subtidal community. The annual mean density of this population was 8.5 ind 5 m−2 and the corresponding mean biomass was 0.19 mg 5 m−2. The overall sex-ratio was statistically in favour of females (F:M=2.1). Brooding females were collected regularly during the whole year and their mean fecundity was estimated at 19.3 ind per marsupium. The demographic analysis of samples suggested that C. goodsir is a trivoltine species. The potential longevity of individuals is less than one year and differs between generations. Using the sizefrequency method, its annual production was estimated at 1.44 mg ash free dry weight 5 m−2 year−1 with a corresponding annual ratio of 7.79.