The life cycle of a brackish-water population of Palaemonetes argentinus, a normally freshwater shrimp from the warm temperate region of eastern South America, was studied in Mar Chiquita coastal lagoon, Argentina. In 1995, recruitment took place in January, followed by two growth periods, the first at the beginning of summer and the second from the end of winter through most of spring. Females reached a larger size than males. Ovigerous females, probably belonging to two cohorts, were found from October 1995 to February 1996, with a maximum number in November (93 %). Their average size and fecundity varied during the breeding period. The sex ratio did not significantly differ from 1:1 except in November. Evidence of mature oocytes in the ovaries was found in ovigerous and postovigerous shrimps, suggesting that they breed twice a year. However, only some females of the first cohort produced a second hatch in 1995. The maximum number of undifferentiated embryos carried by a female was 250; there was a positive relationship between fecundity and size. Larvae were collected together with adults from November to March. The life history is similar to that described in several freshwater populations. Differences are restricted to chronological aspects such as a shorter unimodal breeding period and are probably related to climatic factors, since Mar Chiquita is near the southern limit of the geographic range of the species. Data presented here suggest interannual variations in reproductive patterns that lead, sometimes, to a longer bimodal breeding period.