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Time-series sampling, gut content analysis, gut fluorescence measurements, and experiments on feeding, respiration and ammonia excretion were performed at a fixed station (4.5 m depth) on four occasions (from 1990 to 1997) to evaluate the impact of mysids on phytoplankton and zooplankton in a tropical brackish lagoon. Rhopalophthalmus africana , the dominant species, exhibited marked upward migrations at night. Grazing experiments showed that R. africana did not consume sestonic particles in most cases. Predation experiments showed that R. africana actively consumed zooplankton and selected prey according to their size and/or their abundance. The predation rate increased linearly with increasing prey concentration and did not taper at highest prey concentration. A low metabolic oxygen to nitrogen ratio (O/N, 5–13 at:at) reinforced the assumption of a relatively permanent carnivory. Nocturnal increase in gut fullness suggests night-time feeding, but relatively high gut fluorescence during the day, resulting mainly from a fluorimetric tracing of animal prey, suggested also a diurnal ingestion of zooplankton items concentrated near the bottom. The daily grazing impact of R. africana on phytoplankton was very low (0.14% of the in situ chlorophyll concentration), whereas its daily nutrient recycling through excretion (8–15% of the in situ NH 4 concentration) was rather high. The predation impact (33–154% of the zooplankton production) was high and suggests an important role of mysids to explain the low zooplankton/phytoplankton biomass ratio and the low transfer efficiency observed within the food chain of the western part of Ebrie Lagoon.