ABSTRACT. – The abundance of the invertebrate fauna inhabiting a phanerogam bed of Posidonia oceanica was compared in Caulerpa taxifolia invaded and non-invaded patches in the North-Western Mediterranean in order to select the most sensitive taxonomic groups to C. taxifolia invasion. A total of 10452 individuals were counted in 36 samples of 0.142 m² each, on a sampling period spreading from November 1998 to September 1999. In agreement with the “taxonomic sufficiency” theory, only higher taxonomic groups were analyzed. The Indicator Value Analysis (IndVal method) highlighted that Anomura, Peracarida, Decapoda, Echinoidea, Bivalvia and Gastropoda are the groups most sensitive to C. taxifolia invasion. On the other hand, Ophiuroidea and, to a lesser extent, Holothuroidea were positively affected by C. taxifolia invasion. Our findings are consistent with the hypothesis that C. taxifolia negatively affects invertebrate fauna, strengthening the conclusions of previous studies. The increase in the siltation rate within invaded patches and the decrease in micro-habitat availability are likely to be the main factors explaining C. taxifolia invasion impacts on invertebrate fauna. Despite the extensive colonization of soft-sediments by C. taxifolia worldwide, these impacts have been largely ignored and a large-scale monitoring of the most sensitive groups needs to be carried out in the future.