13C‐NMR studies on the effect of glucose metabolism on arginine hydrolysis in Mycoplasma fermentans cells have been performed using a continuous perfusion technique. With this procedure we were able to show, in the presence of glucose, the rapid accumulation of lactic acid and, in the presence of arginine, the formation of citrulline that is apparently further metabolized. As the accumulation of lactate and the breakdown of arginine were observed in the simultaneous presence of both substrates, it is suggested that the glucose utilization has little or no effect on the deimination of arginine to citrulline.