Knowledge of the biological role of the brown shrimp, Farfantepenaeus californiensis, in coastal lagoons is lacking. This species has been classified as oceanic that may or may not enter coastal lagoons in early stages and if it does, it migrates back to the ocean when it attains a size of 35 mm total length (TL). Recently, the brown shrimp has been caught in the Agiabampo coastal lagoon system, in Sonora/Sinaloa (Mexico), and the objective of this study is to describe the reproductive period of this species in that system. Monthly sampling campaigns were carried out from January to December 2002, during five consecutive days. Tows in 24 geopositioned stations were done during high and low tide using 15-m-wide trawl nets. A total of 27,694 organisms were analyzed, with a male:female ratio of 4:3. Based on the proportion of mature females, we determined two reproductive periods: one from June to July (summer) and another from October to November (autumn); however, the average abundance was 20 org ha–1 in summer and 2 org ha–1 in autumn. The average size of mature females was significantly smaller (P < 0.05) in summer (129.9 mm TL) than in autumn (134.5 mm TL). Size at first maturity was 145 mm TL in both periods. An important result that contributes to the biological knowledge of F. californiensis is evidence that part of the population matures inside this coastal lagoon and presents one important spawn deposition per year.