Abstract: Background and Objective: The mutualistic interaction between microalgae and bacteria when co-culturing aid each other by promoting growth. This work was planned to determine the benefits of co-culturing Chlorella vulgaris with Nitrobacter sp. in terms of enhanced microalgal biomass and lipid production. Materials and Methods: Different growth media viz., Bristol media, synthetic waste water and dilutions of waste water were used for co-culturing for a period of 16 days. Enhancement of microalgal growth in terms of growth rate, biomass, protein, carbohydrates, chlorophyll and total lipid content in microalgae were determined at regular intervals and at the end of cultivation period. Results: Specific growth rate was promoted in co-culture after 4th day of cultivation whereas monoculture has resulted in highest growth rate after 13th day of cultivation. Protein and carbohydrate contents of mono- and co-cultured C. vulgaris were 20.03 and 9.413 μg mg–1, respectively in 50% sewage water. There was a down trend in dissolved oxygen levels when the microalgae were co-cultured with bacteria. Biomass productivity was 0.0371 g L–1/day in monoculture after 16th day of cultivation and was 0.0285 g L–1/day after 4 days of cultivation as co-culture. Highest lipid content of 20.69% was observed in monoculture and the co-culture has resulted in 17.93%. Conclusion: The results indicated that co-culturing of C. vulgaris with Nitrobacter resulted in enhanced growth promotion as evidenced by increased cellular composition and biomass content. This interaction could be utilized in enhancing microalgal biomass, especially by replacing nitrogen fertilizers in the growth medium.