In the terrestrial slug, Limax, eyes are located at the tip of the superior tentacles. This animal has long been believed to show negative phototaxis through tropotaxis, i.e., it compares the two light intensities detected by bilateral eyes to move away from a light source. As one of the possible manifestations of such negative phototaxis, a circling movement has been observed: if one of the superior tentacles is removed, the slugs continuously move in the direction of the removed side. However, there has been no evidence demonstrating that this behavior is actually based on negative phototropotaxis. In this study, we showed that the slugs do not exhibit the circling behavior in the absence of light, and that amputation of the cerebral commissure also diminishes the circling behavior under light. We could detect light-evoked responses during electrical recording from the cut edge of the cerebral commissure. Labeling of the optic nerve with neurobiotin also revealed the presence of the commissural fibers that potentially transmit the light information to the contralateral cerebral ganglion. Our study suggests that the slug's circling behavior is based on phototropotaxis in which the light intensities detected by the bilateral eyes are compared through the cerebral commissure.