The fecundity of Mystus cavasius Hamilton collected from the Brahmaputra river and the Kongsa river was estimated by gravimetric method. The fecundity of the Brahmaputra river fish ranged from 1250 to 23819 with the mean of 10062±704 for the corresponding length of 13.93±0.24cm, body weight of 24.41±1.08g and ovary weight of 1.99 ±0.14g. Whereas the fecundity of fish collected from the river varied from 721 to 44837 with the mean of 11798±1207 for the corresponding length of 15.15±0.24cm, body weight of 24.88±1.09 g and ovary weight of 3.01 ±0.29g. The average number of eggs per gram of ovary was 5064 & 3979 and per gram body weight was 433 & 449 respectively for the fish of the Brahmaputra river and the Kongsa river. The gonadosomatic index (GSI) was calculated to he 8.29±0.47 and 15.63±4.51 respectively for the Brahmaputra and the Kongsa river. The amount of egg produced by a female was more for the fish of the Kongsa river than the fish of the Brahmaputra river. Higher 'n' value of the length-weight relationship w=aLn for the fish of the Kongsa river indicated better living conditions in the Kongsa than in the Brahmaputra river. The relationship between total length and body weight, total length and fecundity, body weight and fecundity, ovary weight and fecundity of the fish were found to be linear both arithmetically and logarithmically. Relationship were highly significant (P<0.01) and the parameters were positively correlated with each other. Ovary weight-fecundity relationship was found to be best fitted (r= 0.95 & 0.94) for the fish of both Brahmaputra river and Kongsa river.