Single crystals of purified homodimeric controller protein from Aeromonas hydrophilia (C.AhdI) have been grown under several different conditions using vapour diffusion. X-ray diffraction data have been collected using synchrotron radiation from crystals of both the native and a selenomethionine (SeMet) derivative of the protein. The native crystal form belongs to space group P21 and data were collected to a resolution of 2.2 Å. Two crystal forms of the SeMet protein have been obtained and were found to belong to space groups P1 and P21; data have been recorded to 2.0 and 1.7 Å resolution, respectively, for the two crystal forms. Three-wavelength MAD data were collected to 1.7 Å for the SeMet derivative crystal, which is isomorphous with the native P21 crystal.