A theoretical solution is presented for the static three-dimensional shape of a fluid having a finite yield strength after it is extruded slowly from a small source on an inclined plane. The solution is compared to the shapes obtained in laboratory experiments in which slurries of kaolin in polyethylene glycol wax were ejected, slowly and in small incremental volumes, onto a slope. This study is aimed at understanding the shape of highly silicic lava flows, such as large rhyolite flows which occur on steep mountain sides or on gentle slopes, and the role of a finite yield strength in governing the slow spreading and the final static shape. The results indicate that it is possible to obtain an estimate of the yield strength of a fluid which is emplaced under such circumstances, using only horizontal aspect ratios and the slope of the underlying ground. ¿ 2001 American Geophysical Union