The fish population of a flat marine area (near Imperia in the Ligurian Sea), depths between 4 and 8 m, colonized by the allochthonous algaCaulerpa taxifolia(Valhl) C. Agardh, was studied during a series of observations from October 1994 to October 1996. Visual censuses and catches using 200 m trammel nets were carried out. Fifty different fish species were observed, Sparidae and Labridae, with 12 and 10 species respectively, were the two most represented families. Maximum Shannon‐Weaver index obtained by censuses was 3.29 and by net catch 3.40. Abundances ofSymphodus tineaandScorpaena porcusseemed to be a characteristic of this community. A comparison with fishing data obtained in a control area withoutC. taxifolia, but colonized by the phanerogamCymodocea nodosa, gave an idea of the changes produced in the fish population as a consequence of the spread of the allochthonous alga. A comparison with Mediterranean rocky environments indicated the existence of some shared qualitative characteristics as well as lower fish densities on theCaulerpaat the Imperia site.