Simplicilliumspecies have a wide host range and an extensive distribution. Some species are associated with rusts, as well as other plant pathogenic fungi and play an important role in biological control. In this study, two specimens ofSimplicilliumwere collected from Chiang Mai Province, Thailand.Simplicillium formicaesp. nov. was isolated from an infected ant andS. lanosoniveumfromOphiocordyceps unilateraliswhich is a new host record. Species were initially identified using ITS gene sequences and confirmed using morphology coupled with phylogenetic analyses of a combined nrLSU, nrSSU, TEF and RPB1 dataset.Simplicillium formicaediffers from other species in the genus by the presence of flask-shaped synnemata and phialides with intercalary nodes.Simplicillium lanosoniveumresembles other collections of the species by its completely solitary, tapering phialides and globose to ellipsoidal conidia which adhere in a slimly head. A key to species ofSimplicilliumis also provided.