Two crystal forms of DNA polymerase from Thermus aquaticus have been grown at room temperature. Rhombohedral crystals (form I) grown from ammonium sulfate solution diffracted poorly to 10 A only and thus are not suitable for X-ray structure determination. Trigonal crystals (form II) grown from polyethylene glycol solution are more suitable for structure determination since their diffraction pattern extends to 2.5 A at cryogenic temperature upon exposure to synchrotron X-rays. They belong to space group P3(1)21 (or its enantiomorph P3(2)21) and their unit-cell dimensions are a = 106.7 and c = 169.7 A, for flash-frozen crystals. The presence of one molecule per asymmetric unit gives a crystal volume per protein mass (V(M)) of 3.0 A(3) Da(-l) and a solvent content of 58% by volume. X-ray data have been collected to 2.7 A Bragg spacing from native crystals.