Chlamydospores of the fungus Thielaviopsis basicola (Berk. & Br.) Ferraris, which had been on infected tobacco roots growing in nonsterile soil for 8 weeks or longer, exhibited perforations in the spore walls. These perforations were visible only when viewed in the scanning or transmission electron microscopes. The perforations extended through the wall to the lumen of the cell and were accompanied by loss of spore viability. Bacteria were often seen in association with the perforations. The perforations and loss of viability were not observed in the chlamydospores that had remained in sterile soil. It is believed that microorganisms are likely responsible for the perforations and loss of spore viability.