Leaf-cutting ants are a very specialized group of ants that cultivate fungus gardens in their nests, from which they obtain food. The current opinion is that the fungus cultivated by leaf-cutting ants digests cellulose. Here we reassess the cellulose-degrading capability of the fungus by using two complementary approaches tested in four Attini species (genera Atta and Acromyrmex): (1) ability of fungus to grow in cellulose; and (2) lignin/cellulose ratio in the refuse material dumped outside the nest, as an indicator of cellulose consumption. We found that (1) the fungus did not grow in cellulose, and (2) the lignin/cellulose ratio was much lower in the ants' refuse than in material digested by cellulose-digesting organisms, such as brown-rot fungus, termites, and ruminant mammals. This evidence strongly suggests the inability of the fungus to degrade cellulose. Therefore, the fungus–ant symbiosis and the ecological role of leaf-cutting ants need to be reconsidered.