The natural history of the fungus-growing ants provides a spectacular example of a symbiotic association of two very different types of organisms. An anthropomorphic description is difficult to resist. The ants are efficient and industrious farmers. Their single crop is a fungus, grown on a substrate of leaves in carefully fertilized, welltended gardens. Virtually every facet of the ants' behavior and life cycle has been shaped by their association with the fungus they culture. A characteristic feature of the ants' gardening technique is the application of their fecal material to the garden and to substrate being prepared for incorporation into the garden. We have established the biochemical significance of this behavior. The fecal material contains proteolytic enzymes which compensate for a deficiency of such enzymes in the fungus. In addition, the nitrogenous components in the fecal material facilitate the initial growth of the fungus. In biochemical terms, then, one can say that the ants contribute their enzymatic apparatus to degrade protein and the fungus contributes its enzymatic apparatus to degrade cellulose. As in the case of so many other natural symbiotic and parasitic associations, the basis is an integration of complementary metabolic capabilities and deficiencies.