Efficacy of N fertilizer in increasing wheat (Triticum aestivum L.) grain protein is well recognized, but effects of P nutrition are unclear. Phosphorus nutrition of wheat in relationship to grain protein was investigated to ascertain differences among cultivars and interactions of P with N and K. Five hard winter wheat genotypes that differed in grain protein concentration were grown at two Kansas locations on Smolan Pachic Agriustolls and Parsons Mollic Albaqualfs soils with three P levels (0, 56, 112 kg P ha−1) as superphosphate and three K levels (0, 112,224 kg ha−1) as KCL to elucidate effects of P nutrition. Hydroponic studies with 0.005, 0.01, 0.05, 0.1, and 0.5 mM NaH2PO4, 5, 10, and 15 mM NaNO3, and 0.05, 0.5, and 5.0 mM KCI determined effects and interactions of P on nitrate reductase activity and soluble protein in seedling leaves. Other hydroponic studies determined effect of 0.01, 0.1, and 1.0 mM NaH232PO4 on grain yield, protein concentration, and 32P distribution. Field application of 56 and 112 kg P ha−1 increased mean grain yield 150 and 314 kg ha−1, respectively, over both locations and decreased grain protein 0.2 and 0.4% , respectively, at one location, an effect attributed to dilution of grain protein. Nitrate reductase activity of hydroponically grown seedlings was highest with 0.1 mM P and lowest with 0.005 and 0.5 mM P, whereas soluble protein was highest at the lowest P level. In plants grown to maturity in hydroponic cultures, 0.1 and 1.0 mM P increased plant tillers, vegetative N and P concentrations, grain yield, and grain protein and P concentrations. Time of application of 32P did not influence grain 32P concentration. We concluded that P fertilizer had no intrinsically detrimental effect on N metabolism and grain protein in wheat, but that it might indirectly affect grain protein concentration by influencing yield.