Grain protein concentration (GPC) is an important component of durum wheat (Triticum turgidum L. var. durum) quality. The 'Langdon'-Triticum turgidum L. var. dicoccoides chromosome 6B substitution line [LDN(DIC-6B)] contains the high GPC quantitative trait locus "QGpc.ndsu.6Bb." We evaluated the effect of this quantitative trait locus on GPC, grain yield, and other agronomic traits in an adapted durum wheat background, verified its location on chromosome 6B and determined its relationship to the GPC locus previously identified. A recombinant inbred population consisting of 110 lines segregating for high GPC was developed from the LDN(DIC-6B) with two doses of adapted germplasm. This population was evaluated in the field for GPC at two locations in 1995, and for GPC, grain yield, and other agronomic characteristics at three locations in 1996. Segregation analysis for GPC showed a bimodal distribution, indicating a single genetic factor or a tightly linked gene cluster controlling high GPC. This high GPC locus was insensitive to environmental conditions. Grain protein concentration was not correlated with plant height, but was loosely correlated with grain yield and heading date. Quantitative trait locus analysis using simple regression and interval mapping procedures identified a locus flanked by Xcdo365 and Xmwg79 on chromosome 6B as having a major effect on GPC. This high GPC locus, which explained up to 72% of the phenotypic variance, accounted for a 15 g kg-1 increase in average GPC. Selection for the two markers flanking the GPC QTL would be highly effective in introgressing this QTL into durum wheat breeding programs.