Abstract
By means of ultrathin sections and electron microscopy new information was obtained concerning the inner structure of hyphae of Penicilllum chryso-genum. The cell wall is thick and homogeneous, although its thickness varies. A number of round or ellipsoidal nuclei were seen in the cross section of the hyphae. The nuclear membrane is thin and the nucleus is filled with small granules or filamentous bodies. The nuclear materials and the cytoplasm are more or less reticular. The scattered cyto-plasmic granules are probably mitochondrial. They are surrounded by a membrane and the inner layer of the membrane exhibits parallel lamellae and the so-called "cristae mitochondriales".