The small GTPase ARL2 (from Mus musculus) and an effector protein, the delta subunit of human cGMP phosphodiesterase (hPDE delta), were coexpressed and copurified from Escherichia coli as a stable complex. Coexpression significantly increased the otherwise low yield of PDE delta production in E. coli. The complex, which contains ARL2 in the activated GTP-bound form, was crystallized in two forms. The first belongs to the monoclinic space group P2(1), with unit-cell parameters a = 48.1, b = 45.7, c = 74.7 A, beta = 94.0 degrees and one complex (39 kDa) in the asymmetric unit. Cryocooled crystals diffract to 2.3 A using synchrotron radiation. The micro-focused X-ray beam at beamline ID13 (ESRF) allowed the use of very small crystals, which helped to overcome twinning and enabled the identification of a molecular-replacement solution. The second form recrystallized from the first one after several months. These crystals belong to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 44.5, b = 65.4, c = 104.4 A and one complex in the asymmetric unit. They diffracted to 1.8 A using synchrotron radiation.