Physical and chemical evidence have led to the assignment of structure Ia (3,20-bis(methylamino)-4-methylene-4-methyl-9,19-cyclopregnan-16-ol) to cyclobuxine, the major alkaloid of the acetone-insoluble portion of the bases of Buxus sempervirens L. The gross skeletal structure for cyclobuxine was indicated by the structures suggested for the products of selenium dehydrogenation (naphthalenes III and VI, phenanthrene VII, and anthracene IV); dehydrogenation of decyclized cyclobuxine (XIa) gave only phenanthrene-related material. Key degradation products in the structure assignment were a conjugated diene (VIIIa) produced by Hofmann degradation; a mixture of cis- and trans-cisoid cyclopentenones (XVIa and b) arising from oxidation followed by facile elimination of methylamine; and the product from the ozonolysis of cyclobuxine followed by alkali treatment of the thus resultant α-aminoketone, the diosphenol XIIIa, which showed additional conjugation in the ultraviolet spectrum. Extracts of Buxus sempervirens L. have been used since ancient times in the treatment of a wide variety of diseases, including malaria and venereal disease. More recently an alkaloidal extract of the plant has been reported to possess antitubercular properties. Previous chemical studies have indicated the multicomponent nature of the alkaloidal extract. We have isolated four alkaloids from the acetone-insoluble portion of the strong bases of B. sempervirens, and have elucidated the structures of the three major bases. This paper reports the structure of the major alkaloid, cyclobuxine (Ia), the first steroidal alkaloid recognized to contain a cyclopropane ring and the first having a substitution pattern at C-4 and C-14 which is intermediate in the biogenetic scheme between lanosterol- and cholesterol-type steroids. The configuration of cyclobuxine and the constitutions of the remaining two alkaloids will be presented in future communications.