The potential value offish silage as a protein source for freshwater fish was assessed by chemical analysis and by incorporation of silage in exprimental diets for Oreochromis niloticus , Fish silage was prepared from a mixture of Nile tiiapia unedible parts,. Sulphuric formic silage was prepared by addition of 1.5% concentrated sulphuric acid and 1.5% concentrated formic acid to the minced mixture of whole Nile tiiapia bodies. The silage was stored for 24 weeks. The chemical composition of fish silage was determined and some of the changes occuring in protein during storage were described. Three diets were formulated in which fish meal was replaced by fish silages (50,75 andl00% silage), Diets were fed to experimental groups of tiiapia for 13 weeks. A significant difference in growth performance and protein productive value were noted between tiiapia fed on 100% fish silage and other treatements; however feed convertion ratio and protein efficiency ratio showed no significant differences between all treatments. The carcass composition parameters were affected by silage in diets. Cost benefit analysis showed that low profit index and high inclusive cost were obtained by the control diet.