This paper presents a theory to account for the main features of the tidal response of an isolated plane fracture as revealed by the water level in an intersecting open borchole. According to the theory the diurnal and semidiurnal tidal constituents (both amplitude and phase) of water level variations depend on the fracture aperture, the orientation (dip and strike) and the radial extent of the fracture, and the compressibility of the asperites. The theory is applied to water level observations made in six open boreholes located in crystalline rock at a test site at the Chalk River Nuclear Laboratories, Atomic Energy of Canada Limited, in Chalk River, Ontario, Canada. The boreholes were uncased and intersected many open fractures so that a definitive test of the theory could not be made. However, in four cases a fracture model could be found which fitted the observations within the experimental error.