Arginine metabolism by mycoplasmas has been reported to be correlated with pathogenicity. Since a clinically important mycoplasma; M. fermentans hydrolyses arginine as an energy source, the arginine metabolism amongst M. fermentans strains belonging to different isolation sites was investigated. Arginine hydrolysis was detected by the increase in NH 4+ concentration, measured using an NH 4+ -sensitive electrode. The method proved simple to use and gave reproducible results. In broth medium, arginine and glucose were both shown to act as energy sources for growth. It was also shown that, in contrast to previous work, glucose inhibited arginine hydrolysis during growth. There did not appear to be a tight coupling between arginine hydrolysis and growth, since there was substantial hydrolysis after growth had ceased.