When Ss in previous research were tested on a color-word interference task, the stimuli were generally said to have been presented in a well-lighted room, or with adequate lighting, or with a good light source, and the like. The present research attempted to find out the difference in performance between (a) Ss performing the task under a level of illumination of 5 ft-c, (b) Ss performing the task under a level of illumination of 150 ft-c, and (c) a control group performing the same task under a level of illumination of 50 ft-c. Results indicated that the Ss of both the 5 and 150 ft-c groups took significantly longer and made significantly more errors than the control group. It was concluded that levels of illumination under which the color-word interference task is performed significantly affect performance.