Abstract: Some aspects of the reproductive biology of freshwater spiny eel, Mastacembalus armatus (Lacepede) has been studied in terms of sex dimorphism, gonadosomatic index, ova diameter and fecundity. The peaks of GSI during May to July for female (10.58±1.51) and male (8.40±1.80) indicate that the fish have only one breeding season during summer. The range of ova diameter was found to vary from 0.100 to 1.00 mm. The estimation of fecundity ranged from 580 to 10980 eggs, the maximum fecundity was observed from a fish measuring 605 mm in length and 355.8 g in weight, and minimum from a fish, 245 mm in length and 34.7 g in weight. The number of ova per gram body weight was 29.38 and the number of ova present per gram of ovary weight was 407.57. The fecundity increased with the increase in length and weight of the fish. The fecundity data was plotted against total length, body weight and gonad weight of the fish. Fecundity- total length and fecundity- body weight gave a better relationship as compared to fecundity- gonad weight relationship.