Four new biologically active diterpenes, trifoliones A, B, C, and D, were isolated from the tuber of Sagittaria trifolia L. together with three new glycosides, sagittariosides a and b and arabinothalictoside. Their stereostructures were determined on the basis of chemical and physicochemical evidence which included the application of a modified Mosher's method and an exciton chirality method. Trifoliones A, B, C, and D exhibited inhibitory effects on the histamine release from rat mast cells induced by compound 48/80 or calcium ionophore A-23187.