A set of 410 river sections were studied to assess Corsican freshwater fish communities affinities with insular characteristics. Coastal rivers in the French Mediterranean region served as a mainland reference. On a large geographic scale Corsican communities show a species impoverishment of about 52% with the lack of all primary freshwater species such as cyprinids in the native community. The number of species in the watershed surface waters is not significantly different from the mainland, however, the low intercept of the relationship species/area relationship is indicative of differences observed between mainland and Corsica. On a local scale, the upstream Corsican community appears similar to the mainland community. But, as distance from the source grows, the presence of euryhaline species become more important in Corsica. These species, coming from the sea compensate the rarity of primary freshwater species. On the aspect of habitat extension, because of the absence of competitors on the island, troutSalmo truttahas clearly enlarged its downstream habitatniche.