The Gram-negative anaerobe 'Dichelobacter nodosus' is the causative agent of footrot in sheep. Different strains of 'D. nodosus' cause disease of differing severities, ranging from benign to virulent. Virulent strains have greater twitching motility and secrete proteases that are more thermostable than those secreted by benign strains. We have identified polynucleotide phosphorylase (PNPase) as a putative virulence regulator and have proposed that PNPase expression is modulated by the adjacent integration of genetic elements. In this study, we compared PNPase activity in three virulent and four benign strains of 'D. nodosus' and found that PNPase activity is lower in virulent strains. We disrupted the 'pnpA' gene in three benign 'D. nodosus' strains and two virulent strains and showed that deletion of the S1 domain of PNPase reduced catalytic activity. In all but one case, deletion of the PNPase S1 domain had no effect on the thermostability of extracellular proteases. However, this deletion resulted in an increase in twitching motility in benign, but not in virulent strains. Reconstruction of the 'pnpA' gene in two mutant benign strains reduced twitching motility to the parental level. These results support the hypothesis that PNPase is a virulence repressor in benign strains of 'D. nodosus'.