The gametogenic cycle and biochemical composition of the carpet shell clam Ruditapes decussatus, in Mersin Bay, Aegean Sea, were determined by monthly sampling from January 2004 to December 2004. Microscopic examination showed that gametogenesis had begun in January, when the water temperature and chlorophyll a value were 14 degrees C and 3.49 mu g L-1, respectively. The first ripe clam (14 %) was appeared in April, when the water temperature had reached 17 degrees C and concentrations of chlorophyll a and total particulate matter were 4.14 mu g L-1 and 14.2 mg L-1, respectively. Spawning period was observed from May to November, most of the clams were in the inactive stage between October and December. Maximum protein values (from 35.67 +/- 1.92 to 41.45 +/- 1.90 %) were recorded in the early and late gametogenesis (Stage 1 and 2). However, the lowest value was obtained in August (22.12 +/- 1.93 %). After this month, protein content began to rise gradually when the clam gonad was in spawning and inactive stage. Glucide and glycogen values were determined at the highest level (3.87 +/- 0.24 % and 3.01 +/- 0.25 %) in April during the first ripe clam appearance. Moreover, lipid patterns were characterized by the presence of three peaks: first in January (6.14 +/- 0.07 %), second in June (6.34 +/- 0.06%) and third in December (6.26 +/- 0.08 %). These observations clearly showed that gametogenesis of R. decussatus in the Mersin Bay were related to environmental factors and biochemical composition.