During August 1994 to February 1995, subbottom temperatures and bottom water temperatures were measured by a long-term temperature monitoring system "Daibutsu" at the TAG hydrothermal mound, Mid-Atlantic Ridge, 26 degrees N. Reliable temperature data were obtained at the base of the central black-smoker complex (CBC) and within the low heat flow zone (<20 mW/m (super 2) ). In the low heat flow zone, subbottom temperatures varied similar to the bottom water temperature, except that its amplitude decayed exponentially and that the phase delayed linearly with increasing depth. Average thermal diffusivity is 6.3X10 (super -7) m (super 2) /s and the heat flow in 28 mW/m (super 2) assuming a thermal conductivity of 0.82 W/m/K. Subbottom temperatures observed at this site indicate that there was no or very little entrainment of seawater during the period of thermal diffusivity estimation. Heat flow at this site increased by about five times just after the drillings of ODP Leg 158 within the low heat flow zone. It indicates that the drillings within the low heat flow zone changed the thermal regime around here. At the base of CBC, the entrainment of seawater into the mound was detected. An estimated downflow rate is about 44 mm/h. The entrainment of seawater is thought to form anhydrite within CBC.