Sufficient oxygen supply was one of the most important culture conditions on RNA production by Bacillus mesentricus niger No. 6021. RNA productivity was markedly increased at a 850-rpm agitation speed and control of the broth-pa at around 7.0, and the maximum yield of RNA (5.7mg/ml) was obtained with a 5-Z jar fermentor. However, insufficient oxygen supply markedly inhibited RNA accumulation. The decrease of the rate of oxygen supply caused the change in the amount and the kind of the products, and 93.6 g of the precipitate obtained under insufficient oxygen supply was found to be Gln.