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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 stab~ lity of environmental parameters and food availability. A yearly reproductive cycle was seen in both communities, with a spawning season at the end of spring that lasted up to several months. Recruitment data, inferred from abundance of post-settlement stages, indicated lack of settlement during the study period in the unstable community, while a major recruitment in summer, with less intense episodes occurring until autumn, was recorded in the stable community. The intensity of recruitment varied over an order of magnitude in the 2 consecutive years monitored. The gonadal indices were higher and the feeding indices were lower …