Alloisoleucine/isoleucine (aIle/Ile) ratios obtained from fossil mollusc shells collected at localities in southwestern Ohio and southeastern Indiana, where they occur in silt beds associated with the Whitewater and Fairhaven tills, indicate a pre-Wisconsinan age for these tills, which had previously been thought to be early or middle Wisconsinan. The aIle/Ile ratios in shells from beneath the buried soil (Sidney soil) and till exposed near Sidney, Ohio, are most similar to values in shells obtained from Illinoian sediments at Clough Creek in Hamilton County, Ohio; Mechanicsburg southwest, Illinois; and Trousdale Mine in Vermillion Co., Indiana. The first well-developed weathering profile in the sequence above the implied Illinoian age silt at the Sidney cut, therefore, probably represents Sangamonian, early and middle Wisconsinan weathering. Molluscs from an organic silt, exposed near the base of the Bantas Fork cutbank section, also have aIle/Ile ratios that are similar to those measured in shell recovered from the silt at the Sidney cut and from the silt inclusion in inferred Illinoian till at Clough Creek. These data indicate that the organic silt is pre-Wisconsinan. Therefore, the Fairhaven Till, which overlies the silt at the Bantas Fork locality, could be pre-Wisconsinan and the weathering profile developed in the Fairhaven Till may be correlative with the Sangamon Soil of Illinois. The New Paris Interstade silt overlies Whitewater Till at the American Aggregates quarry at Richmond, Indiana. Shells from the silt have aIle/Ile ratios that are intermediate between those obtained from inferred Illinoian age sediments at Bantas Fork, Sidney cut, and Clough Creek, and magnetically reversed sediments at Handley Farm, near Connersville, Fayette County, Indiana. These data suggest a pre-Illinoian age for the silt unit and the underlying Whitewater Till.