An entomophthoraceous fungus, Entomophthora planchoniana, was found in populations of the green peach aphid, Myzus persicae, at Kunsan city on June 2, 1998. Occurrence of this species has never been recorded in Korea. Microscopic observations of this fungus are described, and illustrated. Unique characters are that it forms unitunicate muscae-like conidia with $4{sim}6$ nuclei, and is the only Entomophthora species which can attack aphids.