Following the description of a new species of Macrobrachium Bate, 1868 for the Pantanal region, Brazil, the complete larval development of Macrobrachium pantanalense was obtained under laboratory conditions, The larval stages are described and illustrated, and morphological characters are compared with those of M. amazonicum from the Amazon estuary The main differences between the larvae of these two closely related species are found on the rostrum and in the development of the pereiopods. The new species presents the fifth pereiopod fully developed from third zoeal stage, while it occurs only from the fourth zoea in M. amazonicum larvae. In spite of intraspecific variability, there are also conspicuous and consistent differences in the setation of pereiopod 5. Additionally, the carapace of larval M. pantanalense never presents an antennal spine which is found from third zoeal stage of M. amazonicum. The present description and the morphological differences found between the larvae of the two species indicate a speciation process occurring within the neotropical Macrobrachium amazonicum complex.