A physicochemical and complete x-ray structural investigation of the new alkaloid severidine isolated from the epigeal part of Korolkowia sewerzowii Regel has been performed. The crystals were investigated in the form of severidine hydrochloride, C27H43NO5·HCl, rhombic, a=12.642, b=12.882, c=16.621 Å, z=4, space group P212221, at -120°C. The structure was interpreted by the direct method and was refined to R=0.039. The hydrogen atoms were revealed. The absolute configuration was established by means of the Hamilton test. The skeleton of the cation is constructed of bicyclic and tricyclic fragments linked by an ordinary C-C bond. All the rings have the chair conformation. The bond lengths and valence angles are normal. © 1982 Plenum Publishing Corporation.