A newly isolated strain, Penicillium sp. S-22, was used to produce an enzyme that hydrolyses raw yam starch [raw yam starch digesting enzyme (RYSDE)]. The enzyme activity and overall enzyme productivity were respectively 16 U/ml and 0.19 U/ml h in the batch culture. The enzyme activity increased to 85 U/ml by feeding of partially hydrolyzed raw yam starch. When a mixture containing partially hydrolyzed raw yam starch and peptone was fed by a pH-stat strategy, the enzyme activity reached 366 U/ml, 23-fold of that obtained in the batch culture, and the overall productivity reached 3.4 U/ml h, which was 18-fold of that in the batch culture.