The zooplankton of an eutrophic brackish lagoon (Etang de Berre), near Marseille (France) was studied during an annual cycle. It was characterized by large populations of a pelagic copepod (Acartia tonsa ). Grazing on natural particles by this species was investigated. Adults were able to use an extended range of particle sizes, but showed selectivity for particles concentration was observed (14 mu m). Temperature and particles concentration appear to modify the selectivity behavior. Juvenile stages could also catch particles of various size but did not show the same selectivity for the peak particles as adults. Daily food ration was high comparatively to open-sea copepods. Taking into account several results obtained on food ration, egg production and respiration, an energy budget was calculated for two seasons. During summer, this budget was balanced, but during winter, food intake was too low to cover respiration and egg production expenditures. It is suggested that carnivorism could be a complementary way for Acartia to complete its energy need in nature.