Zooplankton distribution in Marennes-Oleron Bay (France) varies during the year and is essentially determined by seasonal rhythms. The whole set of zooplanktonic taxa can be divided into three groups: the first bearing oceanic affinities; the second with estuarine tendencies; and the third more intermediate. Important water flows inhibit the establishment in a specific zone of an autochthonous community (this community is reduced to such species as Acartia discaudata, Acartia grani and Euterpina acutifrons). Zooplanktonic populations reach their maximum abundances at the end of spring and during summer, particularly during spring tide in the inner neritic zone. The estimation of die grazing pressure of copepods (which are the dominant taxa) is variable according to water masses: an abruptly decreasing gradient of pressure of these organisms on algal stock can be seen from die oceanic to the estuarine zones.