Ultrastructural observations of mycelial and tissue phase with dimorphic fungal pathogen Coccidioides immitis were studied by electron microscopy of thin sections. 1. In mycelial phase of C.immitis contains normal cell components such as nucleus, mitochondria, endoplamsic reticulum, intracytoplasmic membrane system, cell wall and cell membrane as observed in the other encaryotic cells. 2. In tissue phase of C. immitis was larger than mycelial phase in cell size and observed much more vacuoles than mycelial phase. 3. In the contrast of mycelial phase of C. immitis, the tissue phase of cells were observed fibril form of capsular layer.