The Amazon River prawn Macrobrachium amazonicum is widely distributed in lentic and lotic environments of South America, and shows different male morphotypes. In the present study, the effect of crowding on the general population structure of this species and its variation over time were evaluated. Prawns were reared in mesocosms consisting of 12 rectangular ~100 m 2 earthen ponds for ~160 d at densities of 10, 20, 40, and 80 prawns m -2 . Prawn density affected both individual and population development. Increased density reduced the size and frequency of the largest male morphotypes and reproductive females, delayed female maturation, and enhanced the asymmetry of the size distribution of individuals, increasing the frequency of smaller prawns. Although mortality was not affected up to 80 ind. m -2 , individual growth rate and reproductive potential decreased at high densities (≥40 ind. m -2 ). Therefore, the ontogeny and population development after metamor- phosis are density-dependent processes. In conclusion, M. amazonicum has a dynamic and density- dependent population structure. This may be due to intrinsic regulatory mechanisms of the species and/or intraspecific competition. It seems that shifts in the sex ratio and the development pattern of male morphotypes are traits which evolved as part of the life strategy to decrease intraspecific com- petition in crowded conditions and to maintain a large population size.