The biomass size spectra in the littoral rocky fish communities in the Medes Island Marine Reserve and three unprotected localities from the NW Mediterranean were studied over a period of two years. A large number of individuals (81 478 in the protected area and 59 536 in the unprotected zones), belonging to 57 species, were censused. Communities were studied in different characteristic habitats (rocky bottoms and Posidonia oceanica sea grass beds). The slopes of the curves for the normalized size spectra showed that fish biomass in rocky areas was nearly constant from the smallest to the largest size classes. That trend was observed in both the unexploited and the moderately exploited areas, suggesting that the removal of large ichthyophagous individuals (e.g. Dentex dentex, Dicentrarchus labrax, Epinephelus marginatus) did not significantly affect the scale of population biomass with size. The pattern was different for the seagrass bed communities, in that the biomass of the larger size classes tended to increase in the protected area, whereas biomass was nearly constant across all the size classes in the exploited area. These results suggest that the communities at seagrass beds could be more sensitive to disturbance, because a large portion of the fish biomass is concentrated in one or a few species (e.g. Sarpa salpa). This study underscores the interest held out by marine reserves as a point of reference for studies on fish community size spectra and point out the existence of dissimilar size spectrum distributions in heterogeneous communities.