The fish communities of seagrass beds around Tulear (Madagascar) and Marseilles (France) are compared, particular attention being given to community structure and resource partitioning. Physical and biotic conditions are more variable in the polyspecific tropical seagrass meadows of the Mozambique Channel than in the monospecific Mediterranean beds of Posidonia oceanica. Around Tulear seagrasses extend from the surface to a depth of 6 metres, and emerge in part at low tide. Composed of 7 species they form a mosaic of habitats. Around Marseilles, P. oceanica beds are found from 0.3 m to 35 m deep ; our study sites being located between 15 and 20 m. Their internal structure is complex and stratified, and they offer a variety of habitats to the fish fauna. The overall species richness of fish is lower around Marseilles (49 spp.) than around Tulear (157 spp.), the greater number of species being found in the polyspecific seagrass meadows on the reef. The fish species characteristic of seagrass beds are both more numerous and more abundant in our tropical study site (30 % of the community) than in our temperate one (10 %). In both locations, species richness, species diversity, fish numbers and fish biomasses are higher by night than by day. The diet fo these fish is very different at these two time periods : by night all species actively feeding are carnivorous, whereas all herbivorous and omnivorous fish, as well as those feeding on sessile invertebrates, are active by day. At Tulear and at Marseilles, the main prey of seagrass beds fish are Crustaceans (53 to 70 %), Fish (17 to 25 %), Molluscs (4 to 10 %) and Polychaetes (1 to 5 %). However, herbivorous and omnivorous fish, and nocturnal zooplancton feeders, are less numerous in the North West Mediterranean than in the Mozambique Channel, whereas there are no sessile invertebrate feeders around Marseilles. Fish diet diversity is therefore higher in tropical waters than in the Mediterranean, particularly with regard to the fish population actively feeding during the day. The potential competition arising from the large number of fish species living in tropical seagrass beds is quite likely avoided in the following three different ways at least : (1) consumption by tropical fish of some foodresources not used in temperate waters, (2) consumption of some very abundant and highly predictable resources by several species, and (3) decrease in niche breadth and dietary overlap of other species. The only herbivorous fish (Sarpa salpa) found in the Mediterranean only lives in the shallow Posidonia beds, not entering the deeper part of the beds. The seagrass browsers are more numerous in tropical waters, but they only represent a small part of the seagrass beds fish fauna. In both locations, the low consumption rate of live seagrasses by fish may be responsible for the lack of toxic secondary compounds noticed in these aquatic phanerogams.