A study of the gametogenic cycle of the carpet shell clam, Tapes decussatus, was undertaken in the inside and outside of Cakalburnu Lagoon, Izmir Bay, Aegean Sea from August 2006 to July 2007. Histological evidence showed that gametogenesis of T. decussatus continued throughout the year and six stages of gonadal development were observed in two sites. In males, except for November in inside and December and January in outside of lagoon ripeness (Stage III) clam was observed during a year. In addition, spawning (Stage IV or Stage V) continued throughout the year in inside and except for February and March in outside. In females, ripe clam (Stage III) was observed similar time with male in two sites. It was observed that Stage 0 (inactive or resting) was only 3.0-5.0% ratio in November, December and January in two sites. The male/female sex ratio for inside and outside of lagoon was 1:1.06 and 1:1.10, respectively (P>0.05).