The Mesopodopsis slabberi (van Beneden 1861) ( Crustacea, Mysidacea ) population of the Westerschelde Estuary was sampled on a monthly basis and fortnightly basis from May 1990 to December 1991. Distribution pattern, density. biomass, number of generations per year, growth, and production of the species were determined. Higher density and biomass of Mesopodopsis slabberi were found upstream. Although there was no clear relationship between the species and environmental parameter, but it was more likely that adult stages were occurred in the low salinity zone. Marked seasonal and spatial changes were also found. The species was recorded with highest density of 55 ind. m-2 of in August and was disappeared in winter when water temperature was below 5°C. The Bhattacharya method was applied to the length-frequency data and revealed three different cohorts of Mesopodopsis slabberi in the Westerschelde. The overwintering generation lived from autumn until the following spring. The spring generation was released in early spring, while the summer generation was born two or three months later. This indicates growth difference of these three cohorts. Larger body sizes can be found in overwintering cohort, as well as seasonal growth oscillation, indicating that growth of this cohort halted in winter. The other two cohorts grew faster and attained maturity at smaller size. Males and females of Mesopodopsis slabberi showed marked difference in growth. Larger females were also found in every sampling dates. More than 75 per cent of production was contributed from spring cohort. Production in the estuary estimated by size frequency methods gave the range of 23-135 mg AFDW m-2 y-1. Least amount of production was contributed from summer generation.
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