1. Carbomoyl-phosphate synthetase in Schistosoma mansoni utilizes L-glutamine as well as ammonia as nitrogen donor but does not require N-acetyl-L-glutamate for the activity. 2. The enzyme activity was inhibited by UDP, UTP, ADP and AMP, among which UDP was the most effective. 3. Aspartate carbamoyltransferase and dihydroorotase were also found and copurified with the synthetase. 4. Relative activities among these three enzymes were 1:30-60:3-8 throughout the purification. 5. These results suggest that the synthetase plays a key role in the control of pyrimidine biosynthesis de novo.