The objective of this study was to identify quantitative trait loci foreconomically important traits in two families segregating an inactive copy ofthe myostatin gene. Two half-sib families were developed from a Belgian Blue xMARC III (n = 246) and a Piedmontese x Angus (n = 209) sire. Traits analyzedwere birth, weaning, and yearling weight (kg); preweaning average daily gain(kg/d); postweaning average daily gain (kg/d); hot carcass weight (kg); fatdepth (cm); marbling score; longissimus muscle area (cm2); estimated kidney,pelvic, and heart fat (%); USDA yield grade; retail product yield (%); fat yield(%); and wholesale rib-fat yield (%). Meat tenderness was measured asWarner-Bratzler shear force at 3 and 14 d postmortem. The effect of themyostatin gene was removed using phase information from six microsatellitemarkers flanking the locus. Interactions of the myostatin gene with other locithroughout the genome were also evaluated: The objective was to use markers ineach family, scanning the genome approximately every 25 to 30 centimorgans (cM)on 18 autosomal chromosomes, excluding 11 autosomal chromosomes previouslyanalyzed. A total of 89 markers, informative in both families, were used toidentify genomic regions potentially associated with each trait. In the familyof Belgian Blue inheritance, a significant QTL (expected number offalse-positives = 0.025) was identified for marbling score on chromosome 3.Suggestive QTL for the same family (expected number of false-positives = 0.5)were identified for retail product yield on chromosome 3, for hot carcass weightand postweaning average daily gain on chromosome 4, for fat depth and marblingscore on chromosome 8, for 14-d Warner-Bratzler shear force on chromosome 9, andfor marbling score on chromosome 10. Evidence suggesting the presence of aninteraction for 3-d Warner-Bratzler shear force between the myostatin gene and aQTL on chromosome 4 was detected. In the family of Piedmontese and Angusinheritance, evidence indicates the presence of an interaction for fat depthbetween the myostatin gene and chromosome 8, in a similar position where theevidence suggests the presence of a QTL for fat depth in the family with BelgianBlue inheritance. Regions identified underlying QTL need to be assessed in otherpopulations. Although the myostatin gene has a considerable effect, other lociwith more subtle effects are involved in the expression of the phenotype.