The enzyme 1-aminocyclopropane-1-carboxylic acid deaminase from the bacterium Pseudomonas sp. has been crystallized using the hanging-drop method. The crystals belong to the orthorhombic space group P2(1)2(1)2(1) (a = 70.0 Angstrom, b = 70.0 Angstrom, c = 355.0 Angstrom). An asymmetrical unit contains two trimer molecules of M(r) = 110,000. The diffraction data have been collected to 3.5-Angstrom resolution. Analysis of the data using the self-rotation function suggests threefold axes within the trimer molecules and a pseudotetragonal arrangement between the trimer molecules in the cell. (C) 1994 Academic Press, Inc.