Females ofScylla serrata attained sexual maturity after reaching 80 mm carapace width and above. Fifty per cent of females at size range 91–100 mm carapace width were sexually mature. The growth rate of abdomen width or length with respect to either carapace width or length was generally at much higher side in mature females than in the immature specimens. A sharp transition at 80 mm carapace width indicated the morphological changes accompanied with the sexual maturity. The relationship between abdomen allometry and carapace measurements was highly significant (P<0·001).Scylla serrata although bred all through the year, exhibited two peaks of breeding, one in between December–March and another in September–November. The sex-ratio in two biotopes varied considerably with the breeding season and size of the female population. The females were found to be in berried state in inshore and off-shore waters of Karwar at 90 mm carapace width and above. Spawning ofScylla serrata was found to occur only in inshore and off-shore but not in backwaters.