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The relation between the abundance of fruits and arthropods, and the diet and feeding behavior of a group of <em>Cebus apella</em> was studied in the Colombian Amazon during 1999. Fruit availability was measured using phenological transects, and the abundance of arthropods by using manual captures. The abundance of both resources varied throughout the year, which allowed establishing three periods of availability: abundant, intermediate, and scarce availability. 708 hours of monkey observations were completed, and using slow scans, it was determined that they had an omnivorous diet, being principally insectivorous &ndash; frugivorous (48.8% - 42.9%). Capuchins consumed 96 plant species, and the monthly diversity of fruits in the diet was not correlated with the fruit availability. During months of resource abundance, both arthropods and fruits were eaten in high quantities, and fruits showed clumped distribution. During fruit and arthropod shortage periods, the diet included a few key fruit species of hard manipulation, such as <em>Cariniana micrantha</em> (Lecythidaceae), and was complemented by vegetable parts. Unlike other studies, during the fruit shortage, the capuchins did not present the higher consumption of arthropods, since these items were also scarce. Capuchins showed an opportunistic diet, and the resources were used in the same proportion in which they were available. For example, the values of fruit selectivity were very low, and the arthropods capture rates were correlated with their availability.</p>