The house fly, Musca domestica L., is a global pest of public health and agricultural importance. The efficacy of conventional management has been waning due to increasing insecticide resistance. A potential management tool is the entomopathogenic fungus, Beauveria bassiana Vuillemin (Hypocreales: Cordycipitaceae) (strain L90), although time-to-death is slower than desired by potential users. This research investigated the effectiveness of three gram-negative bacteria (Pseudomonas protegens Ramette (Psuedomonadales: Pseudomonadaceae) pf-5, Photorhabdus temperata Fischer-Le Saux (Enterobacteriales: Enterobacteriaceae) NC19, and Serratia marcescens Bizio (Enterobacteriales: Enterobacteriaceae) DB11) on house fly mortality when topically applied, compared to B. bassiana. Each pathogen's virulence was measured by injection into adult female house flies or by topical applications to their thorax. All bacterial strains were highly virulent after injection with 1 × 10⁴ colony forming units (cfu), causing fly mortality within 24 h. Beauveria bassiana resulted in high mortality, 3 d postinjection at the high dose of 1 × 10⁴ conidia/μl. Mortality due to topical treatments of P. temperata and S. marcescens was low even at the highest dose of 1 × 10⁶ cfu/μl. Mortality after topical treatments with P. protegens was evident 4 d after application of 1 × 10⁶ cfu/μl. Mortality from B. bassiana was low at 4 d but increased at 5 d. These results imply that P. protegens holds great potential as a biological control agent for incorporation into an integrated pest management program against adult house flies.