Carbon nitrogen metabolism of two fungiFusarium vasinfectum andFusarium udum associated with the wilt of cotton andCajanus cajan respectively has been studied under varying conditions. The hydrogen-ion concentration of the culture media was found to depend on both carbon and nitrogen sources as well as the fungus. Nitrates and ammonium salts have opposite effects on the acidity of culture media, other conditions being the same the consumption of nitrate ions leads to increased alkalinity while the utilization of ammonium ions leads to higher acidity. The utilization of carbohydrates is found to depend on the nitrogen source as well as the fungus, nitrates in general requiring larger amounts. Nitrogen proportion of the mat is also variable. It is found to depend on both carbon and nitrogen sources, the nature of carbohydrate and the age and specificity of the fungus strain. Young cultures generally show a higher percentage of nitrogen than older cultures, irrespective of sources of nitrogen and carbon. Fusaria utilise both nitrate nitrogen and ammonia nitrogen in the presence of sucrose and glucose.