Spherules of C. immitis have been grown in vitro in modified Roessler's medium under CO(2) tension and continuous cultures now maintained for over 18 months. Transformation of hyphae and development of the spherule form have been studied by thin section electron microscopy. Cells of organisms in the hyphal stage have thin (ca. 50 mmicro), apparently structureless walls and a cytoplasmic membrane. Many nuclei, elongated mitochrondria with both transverse and longitudinal cristae, and lipid particles are present. The hyphal wall thickens and the cell transforms into spherules. A large central accumulation of electrontransparent polysaccharide appears in the spherule. The peripheral cytoplasm contains nuclei, each enclosed in a double-layered membrane, mitochondria, and small dense particles. Prior to cleavage the polysaccharide droplets are lost, while mitochrondria become small and spherical. Endospores are formed and liberated when the spherule wall breaks. These begin to grow and repeat the cleavage cycle.