The Neomysis integer (Leach, 1814) (Crustacea, Mysidacea) population of the brackish part of the Westerschelde estuary was sampled on a fortnightly basis from November 1990 to December 1991. Density, biomass, population structure and brood size were recorded. The Bhattacharya method was applied to the length-frequency data for the detection and separation of cohorts. Growth is described both by a generalised von Bertalanffy function incorporating seasonal oscillations in growth. Secondary production was estimated for each cohort using 4 approaches. The seasonal pattern in density and biomass showed 3 peaks: a relatively small yet distinct, peak in early March (30 ind. m -2, 60 mg AFDW m-2) and 2 main peaks in late spring (160 ind. m-2, 225 mg AFDW m-2) and in summer (140 ind. m-2, 125 mg AFDW m-2). Throughout winter, N. integer density remained well below 30 ind. m-2. Three periods of increased reproductive activity and subsequent input of juveniles were found. This suggests that 3 cohorts were produced per year. The overwintering generation lived from autumn until the following spring. The spring generation was born in early spring and lived for about three mo, while the summer generation lived from summer until early winter. The 3 cohorts showed marked differences in their biology. The overwintering generation showed seasonal growth oscillations, larger brood size and a larger size at maturity. Individuals belonging to the other two cohorts generally grew faster, produced less young per female, and attained maturity at a smaller size. Within each cohort, both sexes exhibited different growth characteristics: females generally lived longer, grew faster and consequently became larger than males. The size-frequency, growth summation and removal summation methods yielded comparable production estimates. The annual production was 0.3 g AFDW m -2 yr-1 with an annual P/B ratio of 6. The average cohort P/B was 3. The size-frequency method gave similar results only when applied to the 3 cohorts and to both sexes ...