Minimum and optimum nonbonded contact distances have been determined for nonpolar side chains of amino acids. From the Cambridge Crystallographic Data Base, 110 crystal structures containing valine, leucine, isoleucine, or phenylalanine have been analyzed. Minimum contact distances are as follows: C.C = 3.41, C.O = 3.34, C.H = 2.68, H.O = 2.48, H.H = 2.17, all ± 0.05 A. The preferred interatomic separations are 0.3-0.5 A higher than these. Phenyl rings show preferred orientations relative both to other rings and to oxygen atoms in their environment.