Nine N-phenyl cyclic isourea derivatives of kuninocyclopentanepolyols were prepared conventionally, and assayed for enzyme-inhibitory activity against five sugar hydrolases. Two derivatives derived from lL-(1.2,4,5/3)-5-aminol-C-(hydroxymethyl)cyclopentane-1,2,3,4-tetraol, and two from lD-(1,2,3,5/4)- and DL-(1,4,5/2,3)-5-aminocyclopentane-1,2,3,4-tetraols have been shown to possess strong potency against a-glucosidase and P-galactosidase, respectively.