The present study was designed to evaluate the effect of replacing the protein source of soybean meal (SBM) by tomato seeds by-product meal supplemented with biogenic L. carnitine as a source of methionine plus lysine mixture (TSBPM) at a rate of 100mg/100g diet on growth performance, feed utilization and economic efficiency. Six experimental diets were formulated, control diet (T₀)  contained 45.4% soybean meal.  Tested Diets (T₁ -T­₅) contained 15 to 75% replacing levels (in 15% increments) of soybean meal protein(SBM) by tomato seeds by-product meal supplemented with L-carnitine (TSBPM).  All experimental diets were isonitrogenous (30% CP) and isocaloric (290.0 Kcal digestible energy/100g diet) and P/E ratio of 68.0 mg protein/Kcal gross energy) and each diet was fed to three replicated of fish groups (20 fish for each replicate).  Diets were fed to monosex Nile tilapia (O. niloticus) 6 days/week (twice daily) at 5% of live body weight through 120-days (experimental period).Results of this study showed that, replacement of SBM by tomato seedss by-product meal (TSBPM) up to 60% significantly improved final body weight (BW).  Increasing  the  replacing levels of SBM by TSBPM up to 75%  decreased daily weight gain (DWG), specific growth rate (SGR), total feed intake (FI), feed conversion ratio (FCR) and protein efficiency ratio (PER) with insignificant differences (P > 0.05)  compared with the control group.  These results revealed the possibility of replacing 75% of the high cost SBM by the low cost TSBPM in monosex Nile tilapia diets. Results of body composition indicated that ,fish group fed diet  TSBPM₆₀ gained the highest percentages of protein content and the lower values of ash content, while the replacement of 75% of SBM by TSBPM released the lower percentages of protein, fat and dry matter contents, and higher ash content  in whole fish bodies, with significant differences (P < 0.05) compared with control treatment. From the economic point of view, replacing 75% of SBM by TSBPM in mono sex Nile tilapia diets reduces feed costs by 40.73%.