Entomopathogenic protozoa have a significant role in the host-insect ecosystem, since they persist endemically and act as population suppressors. The European corn borer, Ostrinia nubilalis (Hübner), the spruce budworm, Choristoneura fumiferana (Clemens), several Muscidae, mosquitoes, and stored-product insects, and their protozoa, are examples of naturally occurring systems. Attempts by man to employ protozoa as control agents must be directed so as to make use of existing host-pathogen relationships. Protozoa can be useful as population suppressors in long-term programs, rather than as “microbial insecticides,” and such use is generally more economically feasible. Critical areas of research include methods of survey, identification of disease, and ascertaining the levels of disease incidence necessary for reductions of pest populations below their economic thresholds, as well as identifying the “economic threshold” in terms of modern pest-management concepts.