The Amazon River prawn, Macrobrachium amazonicum (Heller, 1862), is considered as a species with an extremely wide geographic range of distribution, including the Caribbean coasts of northern South America (Venezuela), the Amazon and Orinoco basins, and the upper Paraná and Paraguay drainage systems (southern Brazil, northern Argentina, Paraguay, Bolivia). In the present study, we compare life-history traits of two geographically isolated populations from Brazil: 1, an estuarine population living in the Amazon River delta (AD); 2, a fully limnic inland population from the southern Pantanal plains (Mato Grosso do Sul, MS; upper Paraguay River basin). Various differences were observed: (1) while AD shrimps grow to 12-15 cm body size (females and males, respectively), a maximum of 7 cm was observed in MS; consequently, (2) fecundity is much higher in AD (maximum: 2200 vs 500 eggs per female); compared to AD larvae, those from MS from showed at hatching: (3) significantly larger size (3.2±0.1 vs 2.6±0.1 mm) and (4) higher dry weight (76±5 vs 62±4 µg), containing (5) more protein (31±5 vs 19±2 µg), but (6) less lipid (6.1±0.8 vs 7.7±1.0 µg per larva; 8.5±1.6% vs 14.4±2.8% of dry weight); (7) AD and MS larvae showed conspicuous morphological differences, mainly in the pereiopods, pleopods, and rostrum; (8) while AD larvae develop through 8-11 stages, those from MS pass through a more extended and variable development with numerous mark-time moults; (9) unlike AD larvae, those from MS are able to develop in freshwater (<0.2 ppt); (10) consistent with their lower lipid content, early MS larvae were less resistant against starvation. Altogether, conspicuous differences in reproductive and developmental traits suggest that separate populations of M. amazonicum are in at least an incipient stage of speciation.