Reworking of sedimentary substrates by terrestrial vertebrates, especially hoofed herbivores, has stratigraphic significance comparable to the bioturbation of marine sediments by benthic invertebrates. Environmental analysis of the Pliocene-Pleistocene Koobi Fora Formation in northern Kenya reveals many vertebrate footprints and trackways in fluvial and lake-margin strata. Some beds are completely reworked by trampling of many animals, presumably ungulates, with subsequent disarrangement of primary grain fabric and sedimentary structures. Examination of tracks and game trails in similar modern Kenyan environments, and comparison with those in older sediments, indicate characteristics useful for their recognition elsewhere. Preservation is best in mud and sand interbeds of medium thickness where the animal foot punches out a plug of coherent surface sediment (usually mud) and presses it into underlying units of contrasting lithology (usually sand). Thicker and less coherent muds simply mold the foot. In both situations the track is flat to concave upward with a discontinuous rim that surrounds a low spot where later wind- or water-laid sediments and bone fragments may concentrate. Trampling of coherent surface mud disturbs the ground surface allowing wind and water to remove the loose sand below. thereby creating shallow erosional depressions on the landscape. Heavy trampling in wet interbeds of sand and mud homogenizes the previously distinct layers into a thicker, more massive unit, typically without any obvious tracks preserved. Although we have identified individual tracks of hippo, antelope, and bird (and a seven print trackway of an early hominid), exact taxonomic assignments are not easily made.