The N-2-fixing bacterium, Azotobacter vinelandii, was used both in single culture and in combination with activated sludge culture for the treatment of nitrogen-deficient wastewaters as an alternative to external nitrogen supplementation. Azotobacter-supplemented activated sludge culture removed more total organic carbon (TOC), especially at low initial TN/COD (total nitrogen/chemical oxygen demand) ratios, than the Azotobacter-free culture. Up to 95% TOC removal efficiencies were obtained with synthetic media of TN/COD <4 when Azotobacter was used singly or with activated sludge. The results indicated clear advantage of using Azotobacter in the activated sludge to improve TOC removal from nitrogen-deficient wastewaters.