Proximate compositions of five economically-important fish species from Lake Victoria and Lake Tanganyika, Tanzania were determined using standard methods and procedures of the Association of the Analytical Chemists. Samples of Lates niloticus, Oreochromis niloticus, Rastrineobola argentea, Limnothrissa miodon and Stolothrissa tanganicae were collected during the dry and rainy seasons (December 2012 to March 2013) and (June to September, 2013). Protein contents (17.35-21.44 g/100 g) were significantly (p<0.05) higher in wet seasons and lower (16.13 -19.77 g/100 g) during the dry season in all the species. Similarly, lipids contents were significantly (p<0.05) higher during the wet seasons (1.01 -3.19 g/100 g) and lower (0.79 -1.79 g/100 g) during the dry season in all the species, while carbohydrate (1.89 -4.46 g/100 g) was significantly (p<0.05) higher during the dry season in all the species. The present study showed that these species are good sources of protein in desirable quantities for normal growth, development and as a remedy to nutritional and health related problems. High contents of protein and lipid during wet season make the fish species desirable for consumption during this period. These fish species are therefore recommended to diabetic consumers due to their low carbohydrate contents.