Microbial control of red imported fire ant, Solenopsis invicta Buren, by the entomopathogenic fungus, Beauveria bassiana (Balsamo) Vuillemin, has been inconsistent. Therefore, fungal isolates and precisely controlled soil moisture and composition were investigated as possible limiting factors. For each experiment, ant colonies were collected from the field and placed into 19-liter buckets at room temperature. Fungal inoculation in all experiments was by point-source placement of B. bassiana growing in a single 100 by 15 mm petri dish into the colony; data were collected for 8–10 wk on percentage infection and on ant numbers by a stake-count method and by a population-index rating. Two fungal isolates did not differ in efficacy, and both were inferior to GardStar (permethrin) in killing the ants. B. bassiana reduced ant numbers more effectively in −0.2 bar soil than in −0.5 bar soil, which in turn was better than 0 bar (wet) or −1.0 bar (dry) soil moisture. Even in dry soil (−1.0 bar), however, up to 22.5% of the ants were infected. According to all three parameters, B. bassiana killed more ants in silt (70:15:15, silt:sand:clay) and in sandy soil (15:70:15) than in clay (15:15:70). Results with B. bassiana in clay did not differ significantly from the control.