ABSTRACT: Daily zooplankton samples were obtained from the Etang de Thau, France, a 70 km² coastal lagoon of the northwestern Mediterranean Sea. The survey was monitored at 2 stations inside the lagoon and at 1 station on the seaside during 4 periods in1994. The microzooplankton, towed with a 40 µm mesh size net, included tintinnids, rotifers, anthozoan larvae, and crustacean and mollusk larvae. The tintinnids and rotifers have not been studied in the lagoon yet. Over all the sampling periods, theirmean biomass represented between 26 and 45% of the net collected microzooplankton biomass, which was estimated by image analysis. Their biomass in the central part of the lagoon free of anthropic activity was close to the level observed at the nearbyseaside. However, atthe shellfish culture area, the occurrence of oysters and of epibiotic fauna resulted in a drastic biomass reduction of these taxocoenoses by a factor of 10 compared to values measured in the middle of the lagoon. Samplescollected during the fall indicated well-developed tintinnidand rotifer populations in this season. Inside the lagoon these populations seemed to be already abundant in early spring (March and June samples). An ordination of taxa biomasses showed 2main factors which might have contributed to the organization of the tintinnidand rotifer assemblages: the geographical position and the thermal period. The geographical position integrated the lagoon-sea water exchange. The thermal periodreflected both the populations' development cycles and the environmental constraints. The resulting effects appeared in assemblages structured in a space and time gradient. The capacity of the tintinnidsand rotifers to quickly colonise the habitat(reproduction modes, resistance forms) and to use a wide range of food resources (organic matter, bacteria, pico- and nanoplankton) likely enhances their ability to serve as trophic links between primary and secondary producers. Hence, their function inthe lagoon ecosystem is not minor.