Population structure and growth of freshwater prawn Macrobrachium rosenbergii (de Man, 1879) reared as monoculture in twenty polders of Kuttanad (South India) following modified extensive methods were studied. Stocking densities in polders varied from 15,000 to 60,000 prawns/ha. Density at the time of harvesting ranged from 9,300 to 27,100 prawns/ha. Density dependent variation in the harvested population was discernible in all the polders. At lower densities (15,000 and 25,000 prawns/ha) the proportion of undersized prawns (<50 g) were comparatively low. Morphotypes such as Small Males (SM) had lesser representation in these polders. Likewise, the preponderance of larger Blue Clawed (BC) Males increased in these treatments. The mean weight of prawns was greatly influence by the proportion of SM morphotypes in the final harvest. As a result least mean weight (52.58±4.21g) were encountered in densely stocked polders (60,000 prawns/ha). The net production varied from 257.3 to 835.5 kg/ha/8 months in lower and larger densities respectively. Statistical analysis showed that there existed no difference in terms of production between TP-4 and TP-5. Present study suggest that a stocking density of 35,000 prawns/ha would be more appropriate for monoculture of M. rosenbergii in the polders of Kuttanad.