The development of chlamydospores of Thielaviopsis basicola from hyphal cells involves the thickening and pigmentation of the cell wall. Electron microscope studies showed that membrane-bound cellular bodies appearing in the cytoplasm of differentiating cells developed from dilated cisternae in the endoplasmic reticulum. Within the membrane-bound bodies, vesicles of up to 0.2-μm diameter were observed which contained electron-dense particles. Vesicles resembling those seen in the cellular body were also present in the cytoplasm close to the plasmalemma. In newly formed chlamydospore cells where wall thickening was complete, the cellular bodies showed loss of internal organization, and most of the vesicles disappeared, leaving a structure resembling a vacuole. The cellular bodies were not present in undifferentiated hyphae or in mature chlamydospores.