An F2 chicken population was established from a cross of a broiler sire-line andan egg laying (White Leghorn) line. There were two males and two females fromboth lines in the base population. The F1 progeny consisted of 8 males and 32females. Over 500 F2 offspring from five hatches were reared to slaughter at alive weight of 2 kg at 9 wk of age. Body weights at 3, 6, and 9 wk wererecorded. The DNA was extracted from blood samples, and genotypes for 101microsatellite markers were determined. Data of 466 individuals from 30 familieswere available for analysis. Interval mapping QTL analyses were carried out. TheQTL significant at the genome wide level that affected body weight at two ageswere identified on chromosomes 1, 2, 4, 7, and 8 and a QTL on Chromosome 13influenced body weight at all three ages. Genetic effects were generallyadditive, and the broiler allele increased body weight in all cases. The effectsfor significant individual QTL accounted for between 0.2 and 1.0 phenotypicstandard deviations and the sum of the additive effects accounted forapproximately 0.75 of the line difference in body weight at 6 wk of age. Thelargest single additive effect was on chromosome 4, and the effect ofsubstituting one copy of the gene was an increase in weight of 249 g.Interactions of the QTL with sex or family were unimportant. There was noevidence for imprinting or of two or more QTL at the same location for any ofthe traits.