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This study assessed the physical characteristics, faecal, ammonia and phosphorus waste production rates of mixtures of groundnut, cottonseed, copra and soybean meals as a replacement for fishmeal in Nile tilapia diets. Six experimental diets were formulated such that the oilseed mixtures contributed 80% of total dietary protein. A fishmeal-based diet served as the control. The diets were randomly assigned in triplicates to the experimental tanks, containing 15 juvenile tilapia with a bulk weight of ~1 kg. Ammonia and phosphorus excretion rates of the diets were monitored at 3-hour intervals over 24 h. The pellet characteristics, faecal and nutrient waste production were assessed. The control diet recorded a mean bulk density of 376.00 ±5.19 g L-1 which was lower than the oilseed-based diets which ranged from 550.67 ± 3.06 g L-1 to 614.27 ± 2.08 g L-1. There was a positive correlation (p < 0. 0001; r = 0.93) between the sinking rates of pellets and their bulk densities. The oilseed-based diets recorded 4-45% higher faecal matter production compared to the control diet. Fish fed Diet 3 had the lowest Total Ammonia-nitrogen (TAN) excretion rate of 106.14±1.95 mg kg-1 while the control diet recorded the highest TAN excretion of 162.31 ±4.39 mg kg-1. Accumulated dissolved phosphorus was 64.52-74.09% lower in the oilseed-based diets compared to the control. [ABSTRACT FROM AUTHOR]