Whereas fish assemblages of tropical seagrass beds have been widely studied, those of tropical macroalgal beds are poorly known. To provide information about these assemblages, the fish of two different types of algal beds (dominated by Sargassum sp. and Halimeda sp.) were sampled using visual census in the south-west lagoon of New Caledonia, and were compared to coral reefs. The number of species present on Sargassum beds (202 spp.) and reefs (187 spp.) was more than twice that observed on Halimeda beds (86 spp.). Only 42 species were shared among the three habitat types. Mean species richness and density decreased sharply from reefs (52 species per transect, 6 fish/m2) to Sargassum (35, 1.5) and Halimeda beds (11.5, 0.2). Differences between the two algal beds probably resulted from the higher structural complexity of the Sargassum beds (presence of a higher coral, rocky and algal cover than in Halimeda beds). Pomacentridae greatly dominated fish assemblages on reefs and Sargassum beds whereas Lethrinidae were prevailing on Halimeda beds. Important similarities were observed between the dominant species from reefs and Sargassum beds (e.g., Neopomacentrus spp., Pomacentrus moluccensis, Scolopsis bilineatus), whereas major differences existed with Halimeda beds on which Lethrinus genivittatus was dominant. Species diversity among trophic categories was relatively similar between habitat types but the density and the biomass varied markedly, especially between Halimeda beds and the two other habitats. Territorial or sedentary fishes were dominant on reefs whereas a higher proportion of the large roaming fishes was found on algal beds. This study gives a useful description of the poorly known fish assemblages in tropical algal beds and demonstrates that different algal habitats support different fish assemblages.