Ultrastructural events occurring during the yeast-like to mycelial phase conversion of the dimorphic fungal pathogen Sporotrichum schenckii were studied by electron microscopy of thin sections. Discrete intracytoplasmic membrane systems were formed as early as 6 h after induction of the conversion stimuli. Within 12 to 18 h, a transitional cell was formed from the parent yeast-like cell. Definite streaming of yeast-like cell cytoplasm and mitochondria was noted before septal formation in the intermediate cell. Although the cell wall of the parent yeast-like phase cell is present in two distinct layers, only the inner layer persists upon transitional cell formation. Yeast-like to mycelial phase reorganization of S. schenckii resembles closely that described for Histoplasma capsulatum, Blastomyces dermatitidis, and Paracoccidioides brasiliensis, and it is suggested that yeast to mold transformation of these dimorphic fungi may proceed via a common conversional mechanism.