The reproductive cycle of a wild population of the oyster pearl Pteria sterna living in the Ojo de Liebre lagoon was analyzed from February 2001 to February 2002. The gonadic development was studied both qualitatively (through histological analysis) and quantitatively (through an analysis of the percent follicular area). P. sterna’s gonadic development consists of five stages (undifferentiated, developing, ripe, spawning, and spent). The percent follicular area turned out to be an adequate quantitative indicator of reproductive activity, with significantly higher values in the ripe months (August to September) and with a decrease directly related to spawning. The reproductive cycle of P. sterna is synchronic and is influenced by temperature and food availability. The reproductive season can be regarded to range from October to April, starting when temperature drops. The size at first maturity was determined at 117.1 mm SH and 106.6 mm SH for females and males, respectively. However, the smallest ripe female and male measured 73 mm SH and 26 mm SH, respectively, and both were undergoing the spawning phase. Differences in the reproductive cycle of P. sterna were found between the one reported here and reports for other Gulf of California's localities.