Abstract: Investigations on the fecundity and gonado-somatic index (GSI) of Mystus gulio were carried out from March 1999 to December 2000. Fecundity was determined at different length, weight and ovary ranges of different individual fishes. At different length ranges from 10-20cm of fish showed different fecundity and significantly highest mean fecundity was found 21589 (19394-23481) from 20-22 cm length ranges of fish. Whereas significantly lowest mean fecundity was 11887 (11436-12712) from 10-12cm length ranges of fish. Fecundity was also determined from a wide weight ranges (15-110g) of fishes. Study revealed that significantly highest mean fecundity was found 22546 (19741-23009) from 90-110g weight ranges of fishes. Whereas lowest mean fecundity was 10982 (10831-12481) from 15-20g ranges of fish. Ovary weight ranges from 16-18g showed significantly highest mean fecundity 21378 (19611-22825). This study stated that the fecundity of fish increased with the increase in size, weight and gonad weight as well. The highest GSI values were found in July. Furthermore, GSI showed that M. gulio has a wide spawning season.