In the current study, commercially valuable penaeid shrimp, Melicertus kerathurus, were attained approximately monthly in the Izmir Bay, southern Aegean Sea from May 2004 to April 2005. For each specimen, we measured body length, body weight and ovarian weight, and calculated gonadosomatic indices (GSI). GSI values results pointed out that the GSI in Melicertus kerathurus increased during vitellogenesis (2.03 +/- 0.24%) but sharply decreased after spawning (0.53 +/- 0.35%) to reach levels similar to those in immature females. We also examined oogenesis in Melicertus kerathurus by histological observation of the ovary to determine reproductive cycle for future domestication. The observation on the gonads leads us to conclude that there are five stages of development for the females: (I) previtellogenic; (II) early vitellogenic; (III) late vitellogenic; (IV) mature; and (V) spent. Histological characteristics showed that the best time to capture adult shrimps for breeding purposes was between the months of April and June. The numbers of each oocyte type that were present were also counted, measured, and used to evaluate the following traits: total oocyte number; mean oocytes diameter; and maximum oocyte diameter. Oocyte diameter was significantly different at all stages (P < 0.05) and the largest diameter was observed as 163.92 +/- 4.51 mu m at stage IV.