| Summary: | Leafcutter ants (<i>Atta</i> spp.) are one of the mos t economically harmful pests in agriculture, considered dominant in the Neotropics and South America. Mature colonies of <i>A. mexicana</i> have a great economic impact on Mexico’s agriculture. Microbial agents in the form of biopesticides are an effective component of integrated pest management (IPM) strategies and may present a better alternative to synthetic insecticides. Among the fungi most used as biological insecticides there are <i>Beauveria bassiana</i> and <i>Metarhizium anisopliae</i>. The objective of this research was to evaluate the effect of the entomopathogenic fungi <i>B. bassiana</i> and <i>M. anisopliae</i> of commercial origin and a native strain of <i>B. bassiana</i> from México (MA-Bb1) on adults of <i>Atta mexicana</i> under controlled conditions. In the bioassay, five formulations and a control group were tested (<i>B. bassiana</i> MA-Bb1, <i>B. bassiana</i> MA-Bb1+ Diatomin<sup>®</sup>, <i>B. bassiana</i><sup>®</sup>, <i>M. anisopliae</i><sup>®</sup>, Diatomin<sup>®</sup>, and Tween 80 (0.01%). The MA-Bb1+ Diatomin<sup>®</sup> biopreparation induced the highest mortality (100%) in four-week-old <i>A. mexicana</i>, followed by the MA-Bb1, <i>M. anisopliae</i><sup>®</sup>, and <i>B. bassiana</i><sup>®</sup> biopreparations, which caused mortality of 83.33%, 73.98%, and 68.70%, respectively. Treatments containing <i>B. bassiana</i> and <i>M. anisopliae</i> were efficient in controlling <i>A. mexicana</i> under controlled conditions. The most efficient biological control was achieved with the <i>B. bassiana</i> fungus and Diatomin<sup>®</sup>, which presented the highest total death rate in <i>A. mexicana</i> 96 h post infection, in contrast to the control group (Tween 80), which attained the lowest speed of death in the present investigation.
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