18 pigeons were given nondifferential training to a white vertical line on a red surround. One group was trained with houselight illumination (Group HL); another was trained without houselight illumination (Group NHL). Stimulus control on the angularity dimension was measured with red and with black surround. The relative gradients on the red and on the black surround did not differ significantly for Group HL but for Group NHL the gradient on the black surround was significantly steeper than on the red surround. The relative gradient on the black surround was steeper for Group NHL than for Group HL but on the red surround the gradients did not differ. The results support the hypothesis that stimuli from a more dominant dimension mask control exercised by stimuli from a less dominant dimension.