A data set consisting of 5532 slaughter records collected over three generations from a selection program for increased harvest body weight in the GIFT strain of Nile tilapia (Oreochromis niloticus) was used to estimate genetic parameters and correlated responses in body (live weight, standard length, body width and body depth) and carcass (fillet weight and fillet yield) traits. A multi-trait model using restricted maximum likelihood method was applied to a full pedigree comprising a total of 18,970 animals. The estimates of heritability for body and carcass traits were of moderate magnitude (0.20 to 0.33). The proportions of variance explained by the maternal and common environmental effects were small, ranging from 4 to 8% for body traits and fillet weight, but were negligible for fillet yield. Genetic correlations among body traits were high (0.78 to 0.95), except between standard length and body width (0.56). High genetic correlations (0.78 to 0.96) were also obtained between body traits and fillet weight, whereas those between body traits and fillet yield were generally low (0.35 to 0.44). Genetic changes were measured as the difference in least squares means between the selection and control lines. The correlated increase in fillet weight was 23% in the latest generation studied or 0.97 phenotypic standard deviation units (sP). The correlated response in fillet yield was negligible. In conclusion, selection for high growth significantly increased fillet weight. Strategies for the improvement of fillet yield in the GIFT strain are discussed.