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The goal of this study was to investigate habitat effects on the population structure and condition of the sea urchins Paracentrotus lividus and Arbacia lixula. Population structure (density, biomass, size frequency distribution) of both species has been investigated in two areas along the Ligurian coast, characterised by a different level of human disturbance, representing two alternative states of the infralittoral rocky bottom habitat, the photophilic algae assemblage and the barren ground. Samplings have been carried out in two sites within each area in two times, during Spring 2006. According to this experimental design, 10 individuals per species have been collected to evaluate trophic and reproductive conditions and analyse their gut contents, and, in order to relate feeding conditions to food availability, benthic assemblages have been characterised. P. lividus and A. lixula densities are different, as much as their diets change according to the benthic assemblage the urchins live in, but their trophic and reproductive conditions are not affected by food availability. Our results suggest that top-down processes rule the population structure while bottom-up ones cause changes in the diet, but do not alter the general conditions of the urchins. [ABSTRACT FROM AUTHOR]