Peptide deformylase (PDF) is responsible for cleaving the formyl group at the N-terminus of nascent polypeptide chains in eubacteria and is essential to bacterial cell viability. A recombinant PDF of the thermophilic bacterium Thermus thermophilus HB8 has been crystallized by the hanging-drop vapour-diffusion method using PEG 4000 as a precipitant. The crystals belonged to the tetragonal space group P4(1) or P4(3), with unit-cell parameters a = b = 62.58, c = 105.27 A, and are most likely to contain two molecules in an asymmetric unit, giving a crystal volume per protein weight (V(M)) of 2.3 A(3) Da(-1) and a solvent content of 46.7%.