The cosegregation between a genetic marker and the QTL in a well-designed mapping population is the basis for successful QTL mapping. Linkage disequilibria are, however, also expected among individuals that descended from the same breeding line, and some common haplotypes should carry on and segregate among individuals of the line. These identical by descent haplotypes make it possible to identify and locate the QTL segregating in the line. We report the identification of common haplotypes within commercial lines of Bos taurus and their associations with growth traits. One hundred and seventy six male calves and their 12 sires (9 to 30 male calves of each sire) of the Beefbooster, Inc., M1 line selected for maternal traits over 30 yr were genotyped using 16 microsatellite markers chosen from bovine chromosome 5 for the initial haplotype and growth association analysis. In order to verify the results from the M1 line, another 170 male calves and their 14 sires from the Beefbooster M3 line were genotyped using nine microsatellite markers chosen from bovine chromosome 5. The alleles of each male calf contributed by the sire and by the dam were identified, and haplotypes in the M1 line were established along 93% of bovine chromosome 5. The haplotypes in the M3 line were established along the chosen regions of bovine chromosome 5. Regression analysis detected 10 haplotypes in three chromosomal regions (0 to 30 cM, 55 to 70 cM, and 70 to 80 cM) that showed significant associations with birth weight, preweaning average daily gain, and average daily gain on feed in M1 line and 9 haplotypes associated with the growth traits in the same chromosomal regions in the M3 line at the comparisonwise threshold level. On average, the 19 haplotypes have an effect of 0.68 SD on the growth traits, ranging from 0.41 SD to 1.02 SD The results provide a useful reference for further positional candidate gene research and marker-assisted selection.