Novel experimental techniques to measure ultrasonic velocity and attenuation of surface waves on fluid-filled porous natural rocks are presented. Our experimental results are consistent with the theoretical predictions of Feng and Johnson (1983). Depending on the interface conditions, i.e., whether the surface pores are open or closed, pseudo-Rayleigh, pseudo-Stoneley, and/or true Stoneley surface waves may exist on fluid-saturated rocks. We have demonstrated the existence of the ''slow'' surface wave (true Stoneley mode) on fluid-filled porous rocks with closed surface pores. The velocity and attenuation of the ''slow'' surface mode may be used to assess the dynamic permeability of porous formations. ¿ American Geophysical Union 1994