This study was designed to determine the maximum replacing levels of Fish Meal protein (FM) by a mixture of oil seeds mealprotein (OSM) in five ration for Nile tilapia and Grey mullet. The OSM consisted of cottonseed, sunflower, canola and linseed meals. FM in the basal diet was replaced by OSM in the ration at replacing levels of (control) 0, 25, 50, 75 and 100%. A total number of 60 fingerlings; 40 of Oreochromis niloticus monosex and 20Mugil cephalus per hapa (3*8*1 m-2.5/m3) were randomly distributed into five treatments, each in two replicates. After 6 months of feeding, replacement of 50% of FM by OSM for Nile tilapia and 25% for grey mullet did not significantly affected Feed Conversion Ratio (FCR) and Protein Efficiency Ratio (PER), Crude Protein (CP) and Ether Extract (EE), while the highest
replacing levels (75 or 100%) significantly (P<0.05) reduced these parameters. Growth parameters were relatively parallel to those of FCR and PER whereas, replacement up to 50% exhibited Body Weight (BW), Body Length (BL), Weight Gain (WG) and Specific Growth Rate (SGR) not differing significantly (P<0.05) from
the fish fed control diet. Compared to control, increasing OSM in the ration significantly reduced Hemoglobin, Hematocrit and the activity of Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST). The incorporation
of OSM in ration did not significantly affect whole-body Dry Matter (DM) and Crude Protein (CP).
Based on results obtained in this study and on the economical evaluation, it could be concluded that, replacement of FM by OSM up to 50% in tilapia ration but 25% in mullet ration reduced feed costs/kg diet.