The human burial series from the West Mouth of Niah Cave (Sarawak) offers a unique opportunity to explore prehistoric subsistence patterns in lowland tropical rainforest. Over 200 primary and secondary burials, classified as pre-Neolithic and Neolithic, have been recovered since preliminary excavations began there a half-century ago. Stable isotope ratios of carbon (13C/12C, reported as δ13C values) derived from human tooth enamel provide a quantitative measure of individual food consumption during the time of enamel formation. Such data provide a robust and independent assessment of total diet that complements other subsistence information recovered from the archaeological record. West Mouth human tooth enamel examined shows a broad range of δ13C values ( 15.70 to 11.30), consistent with a C3-based subsistence regime as would be expected in rainforest habitats dominated by C3 vegetation. Pre-Neolithic individuals have more negative δ13C values on average (N = 15, X = -14.3%) than Neolithic individuals sampled (N = 28, X = -13.1%). This isotopic shift is statistically significant and suggests a fundamental change in human subsistence between the late Pleistocene/early Holocene and later Holocene inhabitants at Niah. Pre-Neolithic δ13C values suggest broad spectrum rainforest foraging, whereas less negative Neolithic δ13C values, on average, suggest a more coordinated regime of food production and/or collection. Studies of δ13C variation in rainforest habitats contribute to this interpretation, particularly with respect to the "canopy effect," whereby closed-canopy foraging predicts more negative δ13C values, while food resources consumed by exploiting more open settings (such as fields, gaps, and swamps) predict less negative δ13C values. These data have important implications for interpreting the nature of human subsistence in a rainforest setting prior to and after the potential adoption of agriculture by the inhabitants represented in the West Mouth burial series.