Seasonality in tropical forests can be more subtle than that of temperate forests but still affects the resources available to wildlife. Much work has been done describing changes in fruit availability and dietary composition but the nutritional quality of any particular food item is assumed to be relatively constant. We investigated seasonal changes in the quality of the ripe fruit of Celtis durandii, a common tree that produces fruit year-round and is important in the diets of many species. The lipid content of the ripe fruit was found to be highly variable (0.3–30.8% dry matter) among months and this variation was positively correlated with the summed daily rainfalls of the previous and concurrent months. The amount of this fruit in the diets of three frugivorous primate species (Cercopithecus mitis, Cercopithecus ascanius and Lophocebus albigena) was positively related to measured or estimated lipid levels in the fruit. Such predictable changes in the quality of a constantly available fruit have not been previously reported and suggest that the resources provided by tropical forests may be more seasonal than shown by common measures of fruit availability.