This chapter attempts to summarize ultrastructural findings on the agent of paracoccidioidomycosis. The fungi are not easily amenable to ultrastructure cytological examination due to the chitinous nature of their cell walls. The basic techniques for the preparation of P. brasiliensis intended for electron microscopy are very similar to those applied to other fungi but differed in relation to the modality of ultrastructural study, i.e., transmission, scanning, or freeze-etching. In transmission electron microscopy, mycelia appear to be formed by elongated and branched septated hyphae, and, depending on the angle of section, acquire different shapes. Most of pathogenic fungi are mononucleate. However, P. brasiliensis is a multinucleate organism both in the mycelial and yeast forms. Lipid bodies are recognized as ubiquitous components of pathogenic fungi where they serve as storage material and are also associated with membrane physiology. Mycelial, yeast, and transitional forms of P. brasiliensis have been examined ultrastructurally after their being submitted to antifungal compounds.