the objective of this study was to evaluate the effect of weight categories on the relation of the measures of standard length; length of the head; height of the body measured at the front of the 1° ray of pectoral, dorsal, anal and tail fins; width of the body measured at the front of the insertion of 1o ray of pectoral, dorsal and anal fins; perimeter measured at the front of the insertion of the 1° ray of pectoral, dorsal and anal fins and minor circumference of the peduncle and yield of carcass, filet, head, fin, skin, viscera and residues of the nile tilapia (oreochromis niloticus). eighty tilapia were used, in a completely randomized design, with four treatments (weight categories 250 the 300 g; 350 the 400 g; 450 the 500 g and 550 the 600 g) and 20 replications. each fish was considered an experimental unit. the nile tilapia presented minors variations in the morphometric characteristics in the weight category 450 to 600 g, when compared with the category 250 to 400 g. the weight category 250 the 300 g, 350 the 400 g, 450 the 500 g and 550 the 600 g provided the same yield of carcass, filet and percentages of head, fin, skin and residues. the percentage of viscera in the 550 the 600 g category (10.98%) was significantly lower when compared to 250 the 300 g (12.77%), 350 the 400 g (13.26%) and 450 the 500 g (12.69%). the weight of filet in the 350 the 400 g category (260.50 g) was higher than the 250 the 300 g category (239.95 g). there was no difference in the weight of fillet between the 450 the 500 g category (284.20 g) and 550 the 600 g (292.80 g). the category 450 the 500 g and 550 the 600 g presented higher values of weight of fillet when compared to 250 the 300 g and 350 the 400 g. according to the weight of fillet found, the ideal weight of slaughter of the tilapia is between 450 and 500 g in a range of 250-600 g.