The production of xylanase by Aspergillus awamori was investigated on synthetic medium in shake flasks by the induction of enzyme production by xylan and xylose at different times. No xylanase was formed with xylose as substrate. The highest enzyme activities were obtained with oat xylan added to the medium at the beginning of the cultivation, or after the glucose was consumed. With high spore concentration small pellets were formed, while pellet size increased with decreasing spore concentration. The highest enzyme activities were obtained with filamentous mycelia and small (0.1-0.2 mm in diameter) pellets. With increasing pellet size the enzyme activity in the medium diminished. Phosphate added to the medium was quickly consumed and enriched in the vacuoles of hyphae. Phosphate concentration did not influence fungal morphology. The greatest enzyme activity was obtained at the lowest phosphate concentration.