An F2 cross of male and female chickens from broiler and layer lines was used to detect and map quantitative trait loci (QTL) affecting muscle yields and the relative weights of different muscles. Phenotypic data from 442 individuals in 30 families were analysed by within-family regression analyses using 102 microsatellite markers in 27 linkage groups with genome-wide significance thresholds. Interactions of the QTL with sex or family were unimportant and, for each trait, there was no evidence for imprinting or of multiple QTL on any chromosome. There were 30 significant QTL on 12 chromosomes (chromosomes 1 to 9, 13, 27 and Z) for 11 traits. Significant dominance effects were detected for 10 of the QTL and several were of relatively large effect. The magnitude of each QTL accounted for 3.2–5.7% of the residual phenotypic variation and the additive effects for 0.2–0.8 phenotypic standard deviations (S.D.). The results suggest that there are QTL affecting relative yield of carcass parts and of breast muscle in particular.