Biomass and production of Acartia clausi have been evaluated during two annual cycles (1981-82 and 1984-85) at five stations of a tropical coastal lagoon characterizing the main ecological conditions. Mean annual biomasses lie between 10 and 110 mg DW m^-3 and daily P/B between 0.12 and 0.46. Seasonal variations of biomass vary regionally, but generally higher values are found in the warm dry season (November to May) and the lowest during the humid period (June to October). No seasonal trend is observed for P/B. Lowest P/B values occur in a rural estuarine station whereas highest values are found in that of the urban polluted estuarine zone. Mean contribution of adults to total production is very low (3 to 37 %) while their contribution to biomass is important (20 to 68 %). Biomass variations seem to be mainly related to temperature in the west oligohaline and stable area, and with salinity in the estuarine zone. Food abundance appears to have no great effect on these variations. A predictive empirical model which relates P/B with food abundance, age (stage)-structure, salinity and temperature is proposed. The role of these factors is discussed. It is particularly clear that salinity positively affects biomass and production rate through size and copepodite stage growth rate.