The barnacle Semibalanus cariosus lives between the middle region of the intertidal zone and a line just below the low water level of spring tides. Factors affecting its daily activity were investigated. In the field, few individuals engaged in active cirral beating in the daytime, but many did so at night, thus indicating that daily activity was photoperiodic. In the laboratory, they showed similar periodicity under natural light conditions. While many individuals continued to engage in this activity under dark conditions, few did so under white light during the day. Under artificial dark conditions during the daytime, the number of active individuals increased gradually, but decreased rapidly when they were exposed to white light at night. These results strongly suggest that the activity is directly affected by light. The ecological significance of engaging in this activity at night is discussed.