The crystal and molecular structure of a cobalt(n1) ammine complex of 2-amino-4-imino-2-methylpentanoic acid, [CO(NH,)~{HN=C(CH~)CH~C(CH~)(NH~)(CO,)}] S2O6, has been determined from three-dimensional X-ray data collected by counter methods. The complex crystallizes in the monoclinic space group P24c (C:,, No. 14), with a = 7.576(4), b = 18.209(7), c = 14.068(6) A, B = 125.25(2), Z = 4. The structure was solved by conventional Patterson and Fourier methods, and was refined by block-diagonal least-squares methods to weighted and unweighted 12-factors of 0.041 and 0.040 respectively, for the 1479 unique reflections having I/u(Z) >, 3.0. All hydrogen atoms have been located and refined. The complex cation is essentially octahedral about the central cobalt atom with three amine groups occupying mutually cis coordination sites; the three remaining sites at the cobalt atom are occupied by the tridentate 2-amino-4-imino-2-methylpentanoic acid ligand. The Co-0 distance is 1.914(4) A, while the Co-N(imine) distance is 1.924(5) A. The Co-N(NH3) distances range from 1.968(5) to 1.979(5) A, and are equal within experimental error. Within the tridentate 2-amino-4-imino-2-methylpentanoic acid ligand, the C=N distance is 1.252(7) A, while at the carboxyl group the 'free' C-0 distance [1.233(6) A] is significantly shorter than the distance from the carbon to the coordinated oxygen atom [I .270(7) A]. In the S206 counter anion, the S-S distance is 2.112(2) A, and the S-0 distances range from 1.434(6) to 1.457(4) A, averaging 1.439 A.