ABSTRACT: Density, population structure and spatial distribution of the invasive crab Percnon gibbesi in western Mediterranean waters were investigated. Densities appeared highest within pebble substrates, reaching up to 3 ind. m^-2.Modal length was 20 mm carapace length (CL), with the presence of ovigerous females from 18 mm CL. Maximum densities were observed at 1 m depth. There were significant differences in habitat preference toward boulders of variable size with algal cover nothigher than 0.5 cm. Although individuals were highly active during the daytime, activity patterns were highest at sunset. Further studies are required on the biology and ecology of this grapsid crab to better understand the reasons for its populationgrowth along Mediterranean coasts and to ascertain the means of colonization.