Nosema locustae Canning was selected for intensive development as a microbial agent for controlling grasshoppers because it appeared to offer the greatest potential for success compared to other pathogens of grasshoppers. The criteria used to select N. locustae included (a) virulence, (b) host range, (c) potential for mass production and prolonged storage, (d) adaptability to efficient low-cost application techniques, (e) suitable viability in the habitat of the host, and (f) potential for registration as a microbial insecticide. The development of N. locustae, whether fully successful or not, might be useful as a guide to the development of other protozoa as microbial control agents.