To fully understand the reproductive habitat of brown shrimp, Farfantepenaeus californiensis, this study analyzed reproductive dynamics of the stock in a semiarid coastal region of the Gulf of California. Monthly biological samplings were conducted simultaneously in a coastal lagoon and in adjacent marine waters from January through December 2002, during 5-d periods centered on full moon (full moon ±2 d). An Egg Production Index EPI was used to determine the reproductive period. The main reproductive period was from April to August in coastal waters and June to July inside the lagoon. The EPI was 10 times greater in coastal waters than inside the lagoon. We conclude that, contrary to what is generally believed, brown shrimp can mature and possibly reproduce inside the coastal lagoon, although the most important spawning habitat is coastal waters.