Blueberry maggot (Rhagoletis mendax Curran (Diptera: Tephritidae)) is a pest of blueberries (Vaccinium Linnaeus (Ericaceae)). Tephritid flies, including Rhagoletis Loew species, are susceptible to entomopathogenic fungi, but mortality levels depend on life stage targeted. We tested Metarhizium anisopliae (Metschnikoff) (Clavicipitaceae) strain S54 by application to pupae in the laboratory and using soil drenches in the laboratory and field. We hypothesised that younger (pre-diapause) pupae would be more susceptible to infection than older (post-diapause) pupae. In the laboratory, R. mendax emergence was reduced from 80% in the control to 57–60% with M. anisopliae. Rhagoletis mendax longevity was reduced by two days for both application timings, and mycosed cadavers increased by 9% and 27% with applications to younger and older pupae, respectively, compared to controls. In the field, R. mendax emergence was reduced by 50% with application to younger pupae compared to controls and applications to older pupae. The surfactant Silwet L77 caused reduced R. mendax emergence when pupae were dipped in suspensions. Even though M. anisopliae S54 did not greatly reduce emergence or longevity, infection was successful and younger pupae may be more susceptible than older pupae. Research with other M. anisopliae isolates against multiple life stages should be conducted and effects of soil variables on pathogenicity determined.