The critical concentration of Zn in wheat tissues for the prediction of Zn response and diagnosis of Zn deficiency was examined in a glasshouse experiment with wheat (Triticum aestivum, line QT 4118) grown to anthesis in two Vertisols at Zn application rates of 0, 1.25, 2.5, 5, 10, 15 and 30 kg ha-1 equivalent as ZnSO4 7H2O. The wheat tissues examined were the youngest mature leaf blade (YMB), the leaf immediately below the youngest mature leaf blade (YMB-1), the older leaves, the ear, the stem and the whole tops. The minimum Zn concentration required in a tissue at 0.90 relative yield, referred to as the critical Zn concentration, was determined using the Cate-Nelson graphical and statistical models, the Mitscherlich equation and a two-intersecting straight lines model. The Zn status of wheat was best defined by the Zn concentration in the YMB. Although the critical Zn concentration of the YMB did not vary much with the method of estimation, the Cate-Nelson statistical procedure explained a higher percentage of the variation in Zn concentration in the YMB and relative yield than the Mitscherlich and the two intersecting straight lines models. The critical concentration of Zn in the YMB was 16.0 mg kg-1 dry matter. It is concluded that determination of Zn concentration in the YMB is the best procedure for evaluating the Zn status of wheat plants.