Group I strains of Clostridium botulinum are known to degrade arginine by the arginine deiminase pathway. We have found that C. botulinum Okra B consumed a level of arginine (20 g/liter) higher than the basal requirement for growth (3 g/liter). Arginine was probably the preferred source of nitrogen for biosynthesis but did not serve as a major source of energy. Citrulline and proline were produced as major fermentation products in media containing high levels of arginine, but in media with basal amounts of arginine these products were produced in lower quantities during growth and were later reassimilated. The results indicate that C. botulinum Okra B changes its metabolism during consumption of surplus arginine, and this change is associated with toxin repression, formation of citrulline and proline as end products, and possibly resistance to environmental stresses such as increased acidity and osmolarity.