A recombinant Ia component of Clostridium perfringens iota toxin, which ADP-ribosylates actin, was crystallized by the hanging drop vapor diffusion method using PEG4000 as a precipitating agent. The crystals were obtained in the presence of NADPH, which is similar to a real substrate, NADH, and belongs to the space group P1 (a = 47.9 Angstrom b = 54.5 Angstrom, c = 103.1 Angstrom, alpha = 99.0 degrees, beta = 93.3 degrees, and gamma = 107.2 degrees). The Matthews coefficient of native crystal was 2.7, assuming 2 mol/asymmetric unit. Native data were collected at 2.4-Angstrom resolution. The results from a heavy-atom search showed that lanthanide ions (samarium, holmium) altered the molecular packing, judging from the unit-cell difference. The crystals also belonged to the space group P1 (a = 47.7 Angstrom, b = 53.9 Angstrom, c = 54.6 Angstrom, alpha = 68.9 degrees, beta = 78.3 degrees, and gamma = 73.7 degrees), which is consistent with only one molecule per asymmetric unit.