A twelve day record of temperature collected from the diffuse flow area of a Guaymas Basin hydrothermal site exhibits variations, form a minium of 3.05¿C to a maximum of 4.87¿C, whose periodicity is correlated with tides measured at the nearby town of Guaymas. A simple model, based on the hypothesis that temperature variations result as changes in tidal bottom currents induce changes in the height of the thermal boundary layer, is in good quantitative agreement with observed temperatures for most of the record. The success of this model illustrates that the effects of tidal currents can be strong enough to dominate time variability of a temperature signal at a fixed point in hydrothermal flow. Therefore, tidal currents must be taken into account when using temperature measurements to estimate time varying heat fluxes from hydrothermal diffuse flow regions. ¿ American Geophysical Union 1988