Adenosine monophosphate nucleosidases from Azotobacter vinelandii and Escherichia coli have been studied crystallographically to determine their quarternary structures. Preliminary characterization of the A. vinelandii enzyme shows that the crystals are monoclinic, C2 with a = 347 A, b = 204 A, c = 114 A, and beta = 91.7 degrees. The asymmetric unit contains 12 or 9 subunits of Mr 54,000. Self-rotation functions with data from the AMP nucleosidases from A. vinelandii and from E. coli (Giranda, V. L., Berman, H. M., and Schramm, V. L. (1986) J. Biol. Chem. 261, 15307-15309) are consistent with the monomers arranged as hexamers with point symmetry 32. The hexamers are arranged in the unit cells so that crystallographic 2-fold axes are coincident with the local 2-folds of the point group 32.