A cross between 2 genetically different outcross broiler dam lines, originatingfrom the White Plymouth Rock breed, was used to produce a large 3-generationbroiler population. This population was used to detect and localize QTLaffecting fatness in chicken. Twenty full-sib birds in generation 1 and 456full-sib birds in generation 2 were typed for microsatellite markers, andphenotypic observations were collected for 3 groups of generation 3 birds(approximately 1,800 birds per group). Body weight, abdominal fat weight, andpercentage abdominal fat was recorded at the age of 7, 9, and 10 wk. To studythe presence of QTL, an across-family weighted regression interval mappingapproach was used in a full-sib QTL analysis. Genotypes from 410 markers mappedon 25 chromosomes were available. For the 3 traits, 26 QTL were found for 18regions on 12 chromosomes. Two genomewise significant QTL (P < 0.05) weredetected, one for percentage abdominal fat at the age of 10 wk on chickenchromosome 1 at 241 cM (MCW0058 to MCW0101) with a test statistic of 2.75 andthe other for BW at the age of 10 wk on chicken chromosome 13 at 9 cM (MCW0322to MCW0110) with a test statistic of 2.77. Significance levels were obtainedusing the permutation test. Multiple suggestive QTL were found on chromosomes 1,2, 4, 13, 15, and 18, whereas chromosomes 3, 7, 10, 11, 14, and 27 had a singlesuggestive QTL.