Two populations of Paracentrotus Lividus (Lamarck) (Echinodermata: Echinoidea) from the Northwestern Mediterranean were compared to ascertain the plasticity and adaptive value of certain biological parameters. These populations were found in habitats which differed in terms of stability of environmental parameters and availability of food. The density was higher and more fluctuating in the unstable community. Sigmoidal growth functions were fitted from plate ring analysis, and reflected a higher growth rate in the stable community, resulting in higher mean diameters at equal age. The maximal growth rate was attained in the unstable community at an earlier age than in the stable one. Size-frequency analyses reflected drastic month-to-month changes in population structure in the unstable community, in which the smaller size classes were almost absent. In contrast, at the stable site the distribution was much more uniform through time, and featured a polymodal shape consistent with the development of several annual cohorts. The smaller size classes were the most abundant. It is suggested that different dynamics underlie both populations, the one at the unstable site being driven primarily by episodic storms, which cause high mortalities but carry new individuals to the site. The stable community relies instead on an annual settlement and features a lower and more predictable mortality which allows for the development of a well-structured population.