Acromyrmex lundi and Acromyrmex heyeri are some of the leaf-cutting ants found in Uruguay. These ants are dominant herbivores in the Neotropics and constitute an economically important pest in agriculture and forestry, causing severe defoliations in very young plants. As Metarhizium anisopliae is a well known entomopathogen, the main aim of this study was to evaluate the ability of this fungus for reducing ant populations under field conditions. Metarhizium conidia infect through the cuticle. During the process of infection ants secrete chitinase to digest the cuticle and the hemocele is invaded by the mycelium. When the insect is dead the fungus grows saprophytically and sporulate on the ant body. The median lethal dose of conidia was determined and the successful control of A. lundi and A. heyeri with M. anisopliae under laboratory conditions was achieved. Under field conditions different conidia concentrations of M. anisopliae were assayed and the volume was adjusted according to the nest size, shape   and localization over or under ground. The loss of nest activity after weekly application, during 3 to 4 weeks showed the effectivity of the conidia formulation. As chemical pesticides commonly used for the control of ants will probably be eliminated in the medium term, a transient solution could be the use of conidia and compatible pesticides.