The reproductive period of the blue shrimp Litopenaeus stylirostris off the Gulf of California coast of Sonora and Sinaloa in Mexico has been determined with the percentage of mature females as index. The objective is to determine the reproductive period and output of this species near Agiabampo, Sonora-Sinaloa by combining three indices of mature females; fecundity of size, abundance and size of mature females into a single index, the egg production index (EPI). Monthly samples from January through December 2002 were collected. During five continuous days around the full moon (full moon ± 2 days), trawl nets were used at depths from 1–9 m with a 2 km north and south of the mouth of the Agiabampo lagoon. The proportion of mature females to all sampled females, was highest (66%) in May, mature females were more abundant (13 ± 4 org. ha⁻¹) in July, and the average total length of mature female was significantly smaller (F_(2,291) = 79, P < 0.05) in July than in the others months. Length at maturity was 172 mm. I concluded that it is more important to determine the reproductive processes than only the reproductive period. Particularly off Agiabampo, blue shrimp has a well-defined reproductive output of about 6 × 10⁶ eggs ha⁻¹ with just one massive spawning peak (May to July).