Tylos granulatus is strictly nocturnal，being buried in the sand during the daytme. Emerging from the sand only after dark，it moves about on the surface looking for food through the night. The present paper described on an orienting movement of this animal to a single source of the light hung from the top All the experiments were done at night when the animals are actlve. The animals move about irregularly in a dark field and relatively stralght in a dim light. They show a positive phototaxis to the light of relatively weak intensities Young anmals with 5−9mm body length transferred under the single light go towards the source of the light and move round with the point right under the source of the light as a center of circular paths with a certain radius. This movement is termed as the circus movement. The diameters of the circles are varied with the height of the light source，but not with the light intensities（illumination intensities）. That is，the diameter of the circus movement of Tylosgranulatus increases rectilinearly with an increase in the height of the light source（Figs.7 and 8）. Based on these results，a possible mechanism on the orienting movement，circus movement，to the light source was discussed．