Three new sesquiterpene lactones, dihydro-β-cyclopyrethrosin (3), chrysanin (4), and chrysanolide (5) were isolated from the flowers of Chrysanthemum cinerariaefolium and their structures determined by physical and chemical methods. The configuration at C-1 of pyrethrosin (2) was established as R by its conversion to dihydro-β-cyclopyrethrosin (3), a hydrogenation product of β-cyclopyrethrosin (1), which had the C-1 hydroxyl group assigned by the Horeau procedure and Brewster's benzoate method. The preparation and properties of the C-11 epimers, 12 and 17, of dihydropyrethrosin are reported and their stereochemistry established.