Rainfall-induced crusts on soil have been recognized for some time and their effect on infiltration, erosion, and plant growth pointed out by such investigators as: Duley and Kelly ('39), Schiff and Yoder ('41), Borst and Woodburn ('42), Beutner and Anderson ('43), Ellison ('45), and many others. These investigations without exception have shown that the formation of rain-crust greatly reduces water infiltration, increases runoff, and speeds up erosion. In 1941 Booth ('41) described an algal stratum over hundreds of acres of badly eroded land in the southcentral United States. In contrast with a rain-crust, this algal layer did not slow down the rate of water infiltration and erosion losses were minimized. In the winter of 1944-45 following an extended succession of gentle rains an appreciable darkening of the well-established rain-crust over extensive acreages of desert soil in the vicinity 6f Tucson, Arizona, was noted. A cursory examination of the crust revealed dark, brownish-colored circular colonies of algae. Furthermore, when the crust was peeled from the surface of the soil, its tensile strength had been increased to the extent that pieces as large as 5 or 6 square inches and only 2 to 4 millimeters thick could be handled without disintegration (fig. 1). The underside of these algae-impregnated crusts had a fuzzy appearance with sand grains adhering to what proved to be fungus mycelium.