The ultrastructural changes occurring during germination of spores of Penicillium roqueforti were examined using transmission electron microscopy on thin sections and freeze‐etch replicas. The cell wall of the resting spore was composed of four distinct layers. The outermost layer ruptured and peeled off as the spores became swollen during the initial stages of germination. The germ tube, which emerged from the spore, had a thin cell wall which lacked the outer layers observed in the resting spore. As the germ tube transformed to mycelia it acquired extra outer layers so that the cell wall possessed three layers of material. Germinating spores were multinucleated. Membranous sacs and vesicles and endoplasmic reticulum were observed both in germinated spores and in mycelia.Freeze fracture techniques revealed that the surface of spores was covered with a layer of interwoven fibrils. This technique also confirmed the presence of distinct layers in the cell wall.The capacity of snail digestive enzymes to hydrolyze cell wall material was demonstrate