Bacterial surface motility works by retraction of surface-attached type IV pili. This retraction requires the PilT protein, a member of a large family of putative NTPases from type II and IV secretion systems. In this study, the PilT homologue from the thermophilic eubacterium Aquifex aeolicus was cloned, overexpressed, and purified. A. aeolicus PilT was shown to be a thermostable ATPase with a specific activity of 15.7 nmol of ATP hydrolyzed/min/mg of protein. This activity was abolished when a conserved lysine in the nucleotide-binding motif was altered. The substrate specificity was low; UTP, CTP, ATP, GTP, dATP, and dGTP served as substrates, UTP having the highest activity of these in vitro. Based on sedimentation equilibrium and size exclusion chromatography, PilT was identified as a approximately equal 5- to 6-subunit oligomer. Potential implications of the NTPase activity of PilT in pilus retraction are discussed.