Large differences in the sea urchin Paracentrotus lividus (Lamarck) population structure have been observed in the NW Mediterranean. Differences have been attributed to removal of predatory fish through human fishing activities. This study attempted to determine the effect of predation by fishes on sea urchin population structure. Three infralittoral areas (2 within a marine reserve with a high density of predatory fish, and 1 within an unprotected area with a low density of predatory fish) were studied to compare population structure differences attributable to differing fish predation pressure. P lividus populations were 3 to 4x denser and predation rates were -5x lower in the unprotected site than in the protected sites. Within the reserve, fish accounted for 100% of the predation. P lividus individual mean size was lower within the protected sites than in the unprotected site. Size-frequency distributions showed a negative exponential pattern for P llividus within the reserve, and a bimodal pattern outside the reserve. The urchins showed a crevice-dwelling behaviour in response to the intense predation in the marine reserve. Within the reserve, P lividus population structure appears to be determined by predation by fish. In contrast, in the unprotected area, where the predation rate is much lower, P lividus population structure appears to be determined by recruitment rate. We suggest that a recent increase in P lividus abundance on infralittoral rocky bottoms in the NW Mediterranean, where urchins are not harvested, is caused by human fishing activities. Since P lividus is the major benthic herbivore in the NW Mediterranean, fishing level may, due to cascading effects, determine the structure of benthic infralittoral communities.