Models for the thermal and mechanical evolution of the oceanic lithosphere predict the progressive development of large thermal stresses in the thickening plate. However, there has so far been little direct evidence for the magnitude and distribution of thermal stresses. We present theoretical models which examine the effect of thermal stresses at fracture zones and show that an anomaly of the predicted form can be observed in geoid profiles which cross fracture zones. Specifically, our models predict the development of thermal bending moments which depend on lithosphere thickness or age and therefore change across fracture zones. Including the effect of varying thermal bending moments, thin plate theory predicts vertical, nonisostatic displacements of the lithosphere by plate flexure. The predicted amplitude of the resulting geoid anomaly is large enough to be observed in Seasat altimeter profiles. Furthermore, the general form of this anomaly differs sufficiently from other predicted components of the geoid anomaly at fracture zones to be discernable. The anomaly due to thermal stresses has been clearly identified in geoid profiles across the Clarion and the Udintsev fracture zones. The amplitude of this observed anomaly is well predicted if cooling lithosphere begins to accumulate elastic stresses at a temperature of 700 ¿C, consistent with the maximum depth of seismicity in the oceanic lithosphere. The distribution of thermal stresses with depth is also consistent with focal mechanisms of intraplate earthquakes.