Xanthomonas campestris was investigated in 70 samples of infected plants in the neighbourhood of Luján, province of Buenos Aires, between February and August, 1990. The production of xanthan gum was determined from 50 strains of Xanthomonas campestris, as well as the conversion efficiency of substrate concentration into gum and the number of colony forming units (CFU) of Xanthomonas campestris/ml of broth culture. The highest number of strains producing extracellular polysaccharide was obtained from alfalfa. Xanthomonas campestris pv. alfalfa gave elevated cell concentration and conversion efficiency of glucose in xanthan gum.