A. pullulans was able to produce several morphological growth forms, including yeast forms (blastospores, swollen cells and chlamydospores) and filamentous forms (true hyphae and pseudomycelium). Morphological types were dependent on the chemical composition of the media. In nutrient broth medium, culture contained 71% blastospores and 29% pseudomycelium; in yeast extract (YE) medium, 59% blastospores and 41% true mycelium were observed, but not pseudomycelium. Mutagenic treatment by nitrosoguanidine, ethyl methyl sulphonate and orange acridine induced morphological mutant strains. In YE medium, the ratio between cellular and filamentous shapes of a mutant strain was inverted (78% hyphae versus 22% blastospores) compared to the wild type. At the same time, DNase and RNase activities were 5 to 10 times higher.