The gonad and maturity index cycles of the echinoidParacentrotus lividusand their relations with environmental abiotic parameters are assessed after 2 years of observation in southern Brittany, France. The gonadal cycle is briefly described and eight gonadal stages are characterized. The annual cycle, the time of spawning and the period of gonadal growth are well established, suggesting they are controlled externally. The reproductive cycle has three main phases: the growing phase (late autumn and winter) when gonads accumulate reserve material; the maturation phase (spring and early summer) in which gametogenesis then spawning take place; and the spent/regenerating phase when relict gametes are resorbed by the nutritive phagocytes, the gonads being virtually devoid of sexual cells. The maturity index based on the histological diagnosis of gonads and the use of circular data and polar graphical representation make it possible to reliably determine the spawning period, the rate of gametogenesis and the synchronization of males and females among the echinoid population. From this analysis, we can reasonably say that the gonadal cycle (represented by the gonad index), the rate of gametogenesis, and the end of the spawning period are influenced by temperature whereas the first spawning event appears to be triggered by day length.