The intertidal mite, Bdella septentrionalis Atyeo and Tuxen, inhabits the upper eulittoral and supralittoral of brackish to euhaline: rocky seashores and estuaries. Mites are only active during days and on surfaces wettened by seawater (tidal submerging/spray-water). The activity of B. septentrionalis along the Weser estuary correlates with the tide. In the laboratory the same species, from the same locality, shows a 12-hour periodicity in its activity when expo:sed to constant artificial light. An endogenous circatidal rhythm seems likely. In contrast, B. septentrionalis originating from the Baltic Sea do not exhibit any such activity pattern under the same laboratory conditions. A model for the interaction between tidal activity rhythm and the light-dark cycle is given.