The knowledge of the Bt toxin concentracion within a transgenic plant is basic for the estimation of the performance of the insect pest aimed to be managed with this technology. In a commercial lot at the National Cereal Federation (FENALCE) in the municipality of Granada (Meta), Colombia, samples from plants of transgenic maize YieldGard® were taken to determine the average concentracion of the Cry1Ab toxin. In the laboratory by means of the sandwich ELISA assay, the protein was quantified in vegetative and reproductive tissues with the kit “for Cry1Ab/Cry1Ac Qualiplate Envirologix.” For this purpose, leaves, stem, ear, ear styles, husk, cob, and corn kernels were lyophilized. The analyses indicated that expression fluctuates between tissues from the same plant and between plants in the same plot. The average concentracion of Cry1Ab was: 8.97 μg/g fresh weight in leaves (V7), 8.96 in stems, 2.30 in male inflorescence (VT), 9.57 in unfertilized styles, 7.27 in fertilized styles (R1), 8.39 in cob (R3), 12.19 in husks, and 1.85 μg/g fresh weight in grains (R3). concentracions in seeds and leaves were similar to those obtained in the U.S.A. and Europe. This marked difference in the toxin expression makes Bt corn a plant susceptible to some pests, because there is no uniformity in the Cry1Ab concentracion in the tissues consumed.