Although thalassinidean shrimps exert considerable influence over the structure of benthic communities no ecological studies have been conducted on the larvae and adults of Lepidophthalmus siriboia and Upogebia vasquezi in the equatorial region. The aim of the present study was to identify the reproductive period of these species in an Amazonian estuary. Monthly collections were performed in the estuary of the Marapanim River (northern coast of Brazil) over the course of one year (August 2006 to July 2007) and adults were obtained from bare sandy substrate and between boulders in the intertidal zone of the estuary at four sites. Thalassinidean larvae were also sorted from plankton samples (200 μm mesh) collected monthly at six sites. Larvae of the species Callichirus major, Lepidophthalmus siriboia and Upogebia vasquezi were collected, whereas adults of only the latter two species were found. Upogebia vasquezi was the most abundant species. There was no correlation between larval density and abiotic factors (water temperature, salinity and pH); however, the density of U. vasquezi adults was negatively correlated with salinity in the burrows. Lepidophthalmus siriboia and U. vasquezi reproduce throughout the year in the Marapanim River estuary, with peaks of greater intensity in some reproductive periods of the year, mainly correlated with changes in salinity.