The three-dimensional structure of P2 protein from peripheral nervous system myelin has been determined at 2.7 A resolution by X-ray crystallography. The single isomorphous replacement/anomalous map was interpreted using skeletonized electron density on a computer graphics system. An atomic model was built using fragment fitting. The structure forms a compact 10-stranded up-and-down beta-barrel which encapsulates residual electron density that we interpret as a fatty acid molecule. This beta-barrel shows some similarity to, but is different from, the retinol binding protein family of structures. The relationship of the P2 structure to a family of cytoplasmic, lipid binding proteins is described.