A new crystal form (III) of tobacco leaf ribulosebisphosphate carboxylase [3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39] has been grown by dialysis procedures, and is suitable for structural studies at near atomic resolution. The crystals exhibit birefringence, grow as pseudo-regular rhombic dodecahedrons, and belong to the tetragonal space group P4(2)2(1)2 with a = b = 149 A, c = 138 A, and V = 3.04 x 10(6) A(3). Each unit cell contains two molecules, with two large and two small subunits per asymmetric unit. At low resolution (>10 A) the crystal structure is body centered belonging to space group 1422 with one large/small pair in the asymmetric unit. Thus, at low resolution the molecular symmetry is D(4), the highest possible symmetry for an oligomer of stoichiometry large(8)small(8). Form III crystals may be identical to crystalline inclusions found in chloroplasts.