The sustainability of a given fishery is a function of the number of sexually matured fish present in water. If there is intensive immature fishing, the population of fish reaching the stage of recruitment will decrease, which in turn results in lower yield and biomass. The present study was conducted to estimate the extent of immature fish harvesting by the commercial fishery of Lake Hawassa. Random samples of 962 Oreochromis niloticus and 672 Clarias gariepinus were taken from the fishermen's catch for two weeks from May 15 to 30, 2011, which was peak spawning season for both fish species. The maturity of the sampled fish was determined by visual examination of developmental stages of gonads based on their size, structure and the space they occupy in the body cavity of fish. Lengths at first sexual maturity of male and female C.gariepinus were55.9cm and 54.8cm, respectively and that of male and female O.niloticus were 20.8cm and 20.3cm, respectively. There was heavy immature fishing of C.gariepinus as high as 77.6% but immature fish harvesting was slight for O.niloticus (23.0%). Thus immature fishing of C.gariepinus should be stopped as soon as possible since only one fifth of the population has the chance to breed and replenish the stock before it is caught. Widening the currently used mesh size from 8cm to 10cm is recommended to avoid immature fishing.