Pedigree and marker data from a multiple-generation pig selection experimenthave been analysed to screen for loci affecting quantitative traits (QTL). Pigsfrom a base population were selected either for low backfat thickness at fixedlive weight (L-line) or high live weight at fixed age (F-line). Selection wasbased on single-trait own performance and DNA was available on selectedindividuals only. Genotypes for three marker loci with known positions onchromosome 4 were available. The transmission/disequilibrium test (TDT) wasoriginally described in human genetics to test for linkage between a geneticmarker and a disease-susceptibility locus, in the presence of association. Here,we adapt the TDT to test for linkage between a marker and QTL favoured byselection, and for linkage disequilibrium between them in the base population.The a priori unknown distribution of the test statistic under the nullhypothesis, no linkage, was obtained via Monte Carlo simulation. Significant TDTstatistics were found for markers AFABP and SW818 in the F-line, indicating thepresence of a closely linked QTL affecting growth performance. In the L-line,none of the markers studied showed significance. This study emphasizes thepotential of the TDT as a quick and simple approach to screen for QTL insituations where marker genotypes are available on selected individuals. Theresults suggest that previously identified QTL in crosses of genetically diversebreeds may also segregate in commercial selection lines.