Fic domains in proteins are found in abundance in nature from the simplest prokaryotes to animals. Interestingly, Fic domains found in two virulence factors of Gram-negative bacteria have recently been demonstrated to catalyse the transfer of the AMP moiety from ATP to small host GTPases. This post-translational modification has attracted considerable interest and a role for adenylylation in pathology and physiology is emerging. This work was aimed at the structural characterization of a newly identified Fic protein of the Gram-positive bacteriumClostridium difficile. A constitutively active inhibitory helix mutant ofC. difficileFic was overexpressed inEscherichia coli, purified and crystallized by the vapour-diffusion technique. Preliminary X-ray crystallographic analysis shows that the crystals diffract to at least 1.68 Å resolution at a synchrotron X-ray source. The crystals belonged to the orthorhombic space groupP2₁2₁2₁, with unit-cell parametersa= 45.6,b= 80.8,c= 144.7 Å, α = β = γ = 90°. Two molecules per asymmetric unit corresponds to a Matthews coefficient of 2.37 ų Da⁻¹and a solvent content of 48%.