A study of fossil mots and root casts from East Turkana, Kenya, has shown them to be useful paleoenvironmental indicators. Fossil root casts were obtained from the Pleistocene, Upper Member of the Koobi Fora Formation, deposited within the East Turkana Embayment of the Lake Turkana Basin. Lithofacies and biofacies analysis of the study locality allows the discrimination of several important environments which contain root casts, notably fluvial channels and shallow lacustrine conditions. Five common root cast morphologies have been observed: 1) horizontal, planar root mats; 2) vertical or vertically branching structures; 3) horizontal or horizontally branching structures; 4) diagonally branching structures; and 5) root "balls" (concretionary structures). Root cast morphology is shaped both by the original root morphology and post-mortem, diagenetic development. Line transect surveys of fossil root casts reveal two striking lithofacies-root cast associations. Laminated siltstones of the shallow lacustrine environment primarily contain root mats and horizontal roots, whereas sandstone-conglomerates of the fluvial paleochannels contain predominantly vertical and diagonal root casts, with no root mats. Studies of plant associations in the Lake Turkana area, and work on modern root morphologies suggest a probable explanation for these associations. In arid environments, water availability is a dominant factor in root morphology. Plants growing along intermittent streams or floodplains must frequently rely on perched water bodies or deeper phreatic water, therefore sending out long, vertical tap roots to take advantage of this resource. Plants living in permanent, shallow lacustrine environments or other areas where phreatic water is quite shallow have no such requirement, since their roots are nearly continuously immersed in ground water; during floods the plant stalk itself is subaqueous. The roots of these plants tend to extend laterally in thin mats over large areas. Thus, a study of root cast morphologies may allow for an assessment of paleogroundwater conditions in nonmarine sediments.