When flies were dipped in 1 × 10⁸ conidia/ml conidia suspensions and then kept in the incubator (22 ± 1 °C, 70 ± 5% RH), scanning electron microscope observations revealed that, at 2 h, the majority of adhering Beauveria bassiana conidia were attached to either the wing surface or the interstitial area between the macrochaetae on the thorax and abdomen of the onion maggot adults. Germ tubes were being produced and had oriented toward the cuticle by 18 h. Penetration of the insect cuticle had occurred by 36 h, and by 48 h, germ tubes had completely penetrated the cuticle. Fungal mycelia had emerged from the insect body and were proliferating after 72 h. The superficial area and structure of the wings and macrochaetae may facilitate the attachment of conidia and enable effective penetration. The susceptibility of adults to 12 isolates, at a concentration of 1 × 10⁷ conidia/ml, was tested in laboratory experiments. Eight of the more potent strains caused in excess of 85% adult mortality 8 d post inoculation, while the median lethal time (LT₅₀) of these strains was <6 d. The virulence of the more effective strains was further tested, and the median lethal concentrations (LC₅₀) were calculated by exposing adults to doses ranging from 10³–10⁷ conidia/ml. The lowest LC₅₀ value, found in the isolate XJWLMQ-32, for the adults was 3.87 × 10³ conidia/ml. These results demonstrate that some B. bassiana strains are highly virulent to onion maggot adults and should be considered as potential biocontrol agents against the adult flies.