We investigate the effect of lateral viscosity variations on surface observables by considering an idealized model problem: the motion of a deformable fluid sphere of radius a, density &rgr;s and viscosity &eegr;s at a depth d beneath the surface of a fluid half-space with density &rgr;f and viscosity &eegr;f. We derive a semi-analytical solution for the flow in bipolar spherical coordinates, from which we calculate the surface velocity divergence, the surface topography, and the geoid anomaly above the sphere. Lateral viscosity variations have a large (≥50%) effect on these surface observables when the load is relatively ''hard'' (&ggr;=&eegr;s/&eegr;f>1) and shallow (&lgr;=d/a<1.5), but a small (<15%) effect when the load is ''soft'' (&ggr;≪1) or deep (&lgr;≥2.5). The geoid anomaly produced by a soft upwelling plume differs only slightly (~13%) from that produced by an isoviscous (&ggr;=1) plume. The viscosity contrasts (&ggr;≫1) associated with subducted slabs have a larger effect on the geoid. Geoid models which include such viscosity contrasts may therefore require a greater viscosity increase with depth than existing models which assume isoviscous slabs. ¿ American Geophysical Union 1989