Six amylostatins were isolated from the β-amylase hydrolyzate of crude amylostatins (F-1b fraction) produced by Streptomyces diastaticus subsp. amylostaticus No. 9410. Their structures have been determined, as detailed previously,¹⁾ to be XG, XGG, XGGG, GXG, GXGG and GXGGG, in which G is glucose and X is the compound with C₁₃H₂₃NO₈. They have different inhibitory activities from one another against various amylases. The structure-activity relationship of these amylostatins is discussed in relation to the "subsite theory"²⁾ on the active center of amylases.