Little is known about the phenotypic effects of hypoxia and transient anoxia on the virulence of an entomopathogenic fungus. Conidia of Metarhizium robertsii were produced on: (1) potato dextrose agar medium (PDA) under normoxia; (2) PDA medium under continuous hypoxia; (3) PDA medium under transient anoxia; and (4) minimal medium with lactose (MML) under normoxia. Conidia produced under transient anoxia and produced on MML were the most virulent to Tenebrio molitor. Conidia produced under normoxia and hypoxia were the least virulent. Conidial production and germination speed of conidia produced under normoxia, hypoxia, and transient anoxia were similar; however, MML produced less conidia, but germinated faster than any other treatments.