This study establishes the season for the immigration (recruitment) of Penaeus setiferus and P. duorarum postlarvae into Términos Lagoon, México, as well as the hydrological conditions that occur during this event. The abundance of postlarvae of both species was used to infer the reproductive period for each species. The zooplankton collection was carried out monthly for three years. The nets used were 50 cm in diameter, 1.5 m in length and 505 μm mesh size, and temperature and salinity were measured with an YSI. The fluvial discharges were taken from Comisión Nacional del Agua (CONAGUA). The harmonic pattern variation (HPV) was analyzed for each variable (density of postlarvae, salinity, temperature and fluvial discharge). A cycle was analyzed and divided into two episodes (rise and descent) to estimate the HPV of each postlarval species. The entrance of both postlarval species starts from June to September, with higher density in August, which decreases from October to May, with the lowest density in March. The first episode was established from June to August and the second episode in September. During the first episode the hydrological conditions were: high salinity (34), a warm temperature (30ºC) and low accumulated fluvial discharge (2,828 m3 s-1), while, in the second, they were low salinity (28), a moderately warm temperature (29ºC) and high accumulated fluvial discharge (3,934 m3 s-1).