Field study in tropical dry forest in Tikal National Park, Guatemala, revealed that spider monkeys (Ateles geoffroyi) devoted about 57% of feeding time to fruit (pulp), 20% to the unripe seeds of a single tree species, and the remainder largely to leaves and buds. Fruit, seeds, flowers and leaves ofBrosimum alicastrum (Moraceae) accounted for 56% of the diet, and there was very little predation on invertebrates. A severe drought occurred during 1975, the first year of the study, and a comparison between July of that year with July, 1978 shows that in the drought year there was heavier than normal reliance on fruit of the genusFicus (46% versus 14%) and depressed consumption ofBrosimum (11% vs. 54%). Dietary species diversity (Shannon's measureH) and evenness (Hill'sE) were markedly higher during the drought July than the normal July.