Mangrove forest floors are influenced by the tidal cycle (ca. 12.4-h period). Mangrove crickets (Apteronemobius asahinai) are active on the floors during low tide and rest on stems during high tide. Under constant darkness, most crickets showed a clear bimodal daily pattern in their locomotor activity and the active phases coincided with the time of low tide in the field. The free-running period was 12.56-h on average. Under a 24-h light-dark cycle, the activity was not entrained by the light-dark cycle, but was only suppressed in the photophase. This endogenous rhythm was assumed to be a circatidal rhythm. On the other hand, the circadian component was detected in the circatidal component, i.e. the activity under constant darkness was more intense in the active phase continuing from the scotophase than from the photophase of the preceding light-dark cycle. These results indicate that two clock systems are involved in controlling locomotor activity in mangrove crickets. Furthermore, exposure to an artificial high tide of thirty minutes provided four times at intervals of 12.4-h could entrain the free-running circatidal rhythm under constant darkness. Our study is the first one which clarified the endogenous circatidal rhythm and its Zeitgeber in the terrestrial insects.