Isotopic analyses of mammalian tooth enamel from a well-defined, laterally extensive 150 k.y. interval (9.15–9.30 Ma) reveal an ecological gradient in vegetation on the late Miocene sub-Himalayan alluvial plain. Two contemporaneous river systems deposited the sediments of this interval, with a mountain-sourced system (herein, Blue-gray) to the southwest interfingering with a foothill-sourced system (Buff) to the northeast. Fossil mammal teeth collected from a 32 km transect across this fluvial gradient are significantly more depleted in 13C from northeastern localities than from southwestern localities. This trend occurs in equids, giraffids, suids, sivapithecine hominoids, and anthracotheres. We propose that the Buff fluvial system provided more equably moist substrate conditions and supported more closed-canopy vegetation than the Blue-gray fluvial system. Herbivores living along the paleovegetation gradient thus acquired different carbon isotopic signatures during the period of tooth enamel formation, resulting from higher δ13C values in the forage supported by the Blue-gray fluvial system compared with forage associated with the Buff system. The data also imply that many Siwalik mammalian herbivores displayed marked fidelity in juvenile home ranges and habitats.