From February 1997 to February 1998, random samples of the mollusk Atrina maura were collected on a monthly basis from the Corralero-Alotengo lagoon system, Oaxaca, Mexico. The soft parts were separated from the valve, washed in situ, and placed in a Davison solution. The gonadosomatic index (GSI) and the muscle yield index (MYI) were measured, and the reproductive cycle was characterized by histological cuts. A. maura has two important reproductive periods, one from April to July and another from October to November; there is a resting period from August to September. The reproductive cycle has a direct relationship with the GSI and a reverse relationship with the MYI. There was evidence of a close relationship of the spawning and post-spawning periods with the water temperature (R = 0.991, p > 0.002). Females dominate numerically throughout the year, but the difference is significant (chi2: p > or = 0.05) only in November.