Three Iberian boars were bred to 31 Landrace sows to produce 79 F1 pigs. Six F1boars were mated to 73 F1 sows. The F2 progeny from 33 full-sib families (250individuals) were genotyped for seven microsatellites spanning the length ofchromosome 4. Least squares procedures for interval mapping were used to detectquantitative trait loci (QTL). A permutation test was used to establish nominalsignificance levels associated with QTL effects, and resulting probabilitylevels were corrected to a genomewide basis. Observed QTL effects were(genomewide significance, position of maximum significance in centimorgans):percentage of linoleic acid in subcutaneous adipose tissue (< 0.01, 81); backfatthickness (< 0.01, 83); backfat weight (< 0.01, 80); longissimus muscle area(0.02, 83); live weight (0.19, 88); and percentage of oleic acid in subcutaneousadipose tissue (0.25, 81). Gene action was primarily additive. The Iberiangenotypes were fatter, slower growing, and had lower linoleic and higher oleicacid contents than Landrace genotypes. The interval from 80 to 83 cM containsthe FAT1 and A-FABP loci that have been shown previously to affect fatdeposition in pigs. This is the first report of a QTL affecting fatty acidcomposition of subcutaneous adipose tissue in pigs and provides a guide for themetabolic pathways affected by candidate genes described in this region ofchromosome 4.