The insect-pathogenic fungal genus Strongwellsea (Entomophthorales) contains specialist species infecting only adult flies from cyclorrhaphan adult flies. Flies with infection develop one, two or even three abdominal holes, through which conidia are discharged while the fly is still alive. We describe three new species from Strongwellsea infecting adult flies from the family Anthomyiidae. Strongwellsea caulis infects cabbage root fly, Delia radicum, Strongwellsea lata infects Anthomyia liturata, and Strongwellsea potentia infects Hylemya vagans. The descriptions of the three new species are based on a) pathobiology as revealed by natural host species, b) the morphologies of primary conidia and resting spores, and c) DNA as revealed by ITS2 sequence analysis. Of particular importance is that the two pest species cabbage fly D. radicum and bean seed fly Delia platura, are infected by different species from Strongwellsea, namely S. caulis and S. castrans respectively. Our findings suggest the existence of a high and still little known diversity of highly specialized Strongwellsea species infecting adult flies from Anthomyiidae.