A laboratory study is described in which the engineering properties of subgrade soils were modified by treatment with fly ash derived from low-sulfur western coal. A wide range of soil types including gravel-sand, silty sand, sandy silt, and highly plastic clay exhibited substantial improvements in compressive strength, resilient modulus, and wet-dry and freeze-thaw durability when treated with fly ash from several Wyoming power plants. Application of the test results to design of aggregate-surfaced secondary roads indicates that use of fly ash stabilization can decrease the required aggregate thickness substantially. Currently there are no standards which specify minimum required improvements in the engineering properties of fly ash treated soils.