The present study explored the effects of male-female interactions on the behavior and phenotypic expression of giant freshwater prawn Macrobrachium rosenbergii. Juvenile male (♂) and female (♀) prawns were randomly assigned into three treatments having different M and ♀ sex ratios and stocking densities such as T₁ - only ♂ (1, 3 and 5); T₂ - single ♂: multiple ♀ (1:1, 1:3 and 1:5) and T₃ - multiple ♂: single ♀ (1:1, 3:1 and 5:1). Results showed that males in 1♂: 1♀ occupied significantly more territory (P<0.05) than 1♂, while, males in 5♂ treatment showed significant territory occupation (P<0.05) than males in 1♂: 3♀, 1♂: 5♀ and 5♂: 1♀. In female choice trials, males kept without females (1♂, 3♂ and 5♂) were chosen by the test females significantly (P<0.05) rather than males reared with females in different ratios. The phenotypic analysis revealed that males in 1♂:1♀, 1♂: 3♀ and 5♂: 1♀ treatments were significantly larger (P<0.05) than males in 3♂ treatment. The analysis also showed that males in the 5♂ treatment had significantly longer 2^nd chelate-leg (P<0.05) than males in the 3 ♂: 1 ♀ treatment. This study would provide some baseline data for this carnivorous crustacean, which can be successfully cultured by maintaining the sex ratio.