RESUMEN
The neotropical species Macrobrachium amazonicum (Heller, 1862) is considered a particularly successful species, showing an extremely wide range of distribution (ca 4.000 km across). Populations assigned to this species live in estuaries along the northern and northeastern coasts of South America as well as in fresh water habitats in the Amazon, Orinoco and Paraná-Paraguay river basins. Following recent comparative studies that showed differential ecological, reproductive, developmental and physiological traits in geographically separated populations from the Amazon delta and the Pantanal region, Brazil, we examined the morphology of adult shrimps from these two regions. Based on significant differences, we conclude that the Pantanal population constitutes a new species, which is described here as Macrobrachium pantanalense. The main differences between M. amazonicum and the new species have been found in the morphology of the second pereiopod, the telson, and in the color patterns of both males and females. A modification on the key of American species of Macrobrachium is provided to accommodate the new species.