In this section, we shall discuss 3 genera of actinomycetes: Actinomyces, Nocardia, and Streptomyces. These organisms have been shown to be higher bacteria, but they were thought to be fungi for many years because they havefilamentous forms, 0.5 to 0.8 µ in diameter, which appear to branch. Some species form aerial mycelia in culture. The clinical manifestations of infection are similar to those of a systemic fungal infection. It is now clear that they are not fungi but are closely related to the mycobacteria.