This study examined fecundity, by means of the gonad index of the most common and abundant echinoderm species (Paracentrotus lividus) of the Aegean Sea (western Turkey). Paracentrotus lividus samples were collected monthly at the coast of Cesme for six months and biometric parameters were measured and then the animals' dissected. Gonad indices were calculated. The mean fecundity (number of eggs) was determined and eggs diameter were measured. Mean egg diameter was 91.14 +/- 1.46 mu m. There was a peak in fecundity in March (346.582 eggs/female) that correlated with the observed peak in GSI (11.02 +/- 0.54 %) and egg size (95.68 +/- 01.98 mu m). The results presented here show that a significant different was found in egg numbers and egg sizes between months (p<0.05).