Crystallization and crystal structure of the beta phase of oleic acid (cis-9-octadecenoic acid) have been investigated with morphological observation, X-ray diffraction, and DSC (differential scanning calorimeter). The morphology of growing crystals of the beta phase depends significantly on supercooling. It was found that the beta phase could be classified into at least two solid modifications, beta(1) (mp 16.3 degrees C) and beta(2) (mp 16.0 degrees C). The crystal structure analysis of the beta(1) phase has been performed. The unit cell belongs to a triclinic system of P1, and the asymmetric unit contains two crystallographically independent molecules, A and B. The molecular layer is a unique interdigitated structure, where the methyl group of molecule A and the carboxyl group of molecules B (or vice versa) are located on the same plane. For the conformation around the cis-C=C bond, both molecules approximate to trans-cis-trans conformation, the first case for a cis-monounsaturated fatty acid. The methyl and carboxyl side chains together form a TII subcell. On the basis of its crystal structure, it was speculated that the beta(1) phase has a unique surface structure at the (001) face. The factors for the characteristic properties of the beta phase were also discussed.