The earthworm fauna of the western United States is an amalgam of native and introduced elements. While the native species are mostly members of the family Megascolecidae, and closely related to those of Australia and Southeast Asia, the introduced species are, at least in the North, members of the family Lumbricidae, and mostly native to Europe, though some, Bimastos parvus and B. tumidus, have come from eastern North America. This present work stems from a need, identified by the U.S. Environmental Protection Agency, to determine the potential toxicity of various hazardous wastes to soil organisms. Earthworms, because they burrow through and ingest the soil, are a determining factor in the aeration, drainage, and fertility of moist soils, making them an important ecological component of the soil system.