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The spatial and temporal patterns of recruitment of juvenile clams Ruditapes decussatus were investigated in the Mediterranean lagoon of Thau. The periods of release of clam larvae were determined by monitoring the gonad maturity index in samples of adult females. Two massive spawnings were detected. Recruitment, deduced from spat density, was abundant in one part of the lagoon (Etang des Eaux-Blanches) in 1987 but not in the following years. Conversely, it was absent in another part of the lagoon (Grand-Etang) in 1987, although it occurred previously there. The spatial patterns of recruitment in 1987 were highly heterogeneous. They could be related to circulation, modelled for the periods following spawning. In the Grand-Etang, where no recruitment occurred in 1987, the larvae were confined to the vicinity of intensive shellfish culture zones where they presumably were depleted from the water. Episodic local anoxia also likely caused massive mortality among recruits. Recruitment of invertebrates with planktonic larvae in Mediterranean lagoons thus depends on meterological conditions which directly or indirectly determine the timing of spawning, the circulation patterns, and the occurrence of anoxias. In the lagoon of Thau the presence of large areas of intensive shellfish culture is a potential factor of massive mortality for larvae.