Y-tube tests of phototactic behavior of house flies (Musca domestica L.) involving wild-type, yellow-eyed, and hybrid individuals were conducted under low (8 ft-candles, 86.1 lux) and high (1600 ft-candles, 17,222.8 lux) light intensities. The Y-tube design utilized either a clear or a dark stem leading to the branches where the photochoice was made. No significant differences were found in the responses of the wild-type and hybrid lines to either light intensity or Y-tube design. Significant differences were recorded for the yellow-eyed individuals only under high light intensities, whereas their behavior under low light conditions was indistinguishable from that of the wild phenotypes. Yellow-eyed flies showed a 68.6% photopositive response when tested in dark-stemmed Y-tubes under 1600 ft-candles intensity as compared to the 86.0% and 83.3% photopositive responses of the wild and hybrid lines, respectively. However, the yellow-eyed flies tested in the clear-stemmed Y-tubes under 1600 ft-candles showed only a 28.7% photopositive response--a clear reversal toward photonegativity. Light-dark adaptation of the highly sensitive yellow-pigmented eye while in the stem of the Y-tube is responsible for this switch toward photonegative behavior.