Two forms of the archaeal intron-encoded site-specific endonuclease I-DmoI, namely I-DmoIc and I-DmoIl, have been purified and crystallized. Crystals of I-DmoIc are rod-shaped and diffract to 3.0 A resolution, but further analysis was hampered by twinning. Crystals of I-DmoIl, which is a six-amino-acid C-terminal truncation of I-DmoIc, are plate shaped and belong to space group C2 with cell parameters a = 93.72, b = 37.03, c = 55.56 A, beta = 113.4 degrees, with one molecule per asymmetric unit (Vm = 2.01 A3 Da-1). The crystals diffract to at least 2.3 A resolution. A complete native data set has been measured and structure determination is on-going.