Five Pleistocene loess units in the Mississippi Valley that contain terrestrial gastropod fossils are differentiated by amino acid ratios (aIle/Ile values) in the total acid hydrolysate of the fossil terrestrial gastropods Hendersonia occulta, Catinella and Succinea. Late Wisconsin Peoria Loess, which has been independently dated by radiocarbon and thermoluminescence methods, is identified by alle/Ile values in fossil gastropods which progressively increase from northern Illinois to southern Louisiana. Early(?) to middle Wisconsin Roxana Silt is characterized by alle/Ile values in Hendersonia only slightly greater, and values in Catinella and Succinea distinctly greater than those of the Peoria Loess. AIIe/Ile values in Hendersonia suggest that the "Chinatown silt" (informal term) in Illinois and the Loveland Silt in Arkansas are the same age. Independent geologic evidence suggests that they predate the Sangamon (last interglacial). AIle/Ile values in Hendersonia from the "Sicily Island loess" (informal term) in Louisiana indicate that it is significantly older than the Loveland Silt and the "Chinatown silt". The oldest loess unit recognized by alle/Ile values in Hendersonia from the Mississippi Valley is the "County Line silt" (informal term) in Illinois. The study shows that amino acid ratios in fossil gastropods from loess are an extremely valuable relative-age indicator and provide a means of long-distance correlation of midcontinent loess deposits.