Microcosm experiments were conducted to determine the fractionation of stable carbon isotopes during biodegradation of naphthalene. The microcosms were performed under aerobic conditions, anaerobic (amended with sulfate, amended with nitrate and with no amendments) and sterile controls. The liquid phase was analyzed to determine naphthalene concentration and stable carbon isotope signature. Aerobic microcosm showed that naphthalene degraded aerobically within 60 hours. The δ¹³C increased from -25.5‰ to -25.1‰ (enrichment of 0.4‰ ± 0.08‰) in a single sample in which 95% of the naphthalene was biodegraded. Anaerobic microcosms show that after 288 days, the microcosms with no amendments, amended with nitrate and amended with sulfate had consumed respectively 30%, 50% and 60% of naphthalene on average, compared to control microcosms. Under the denitrifying conditions, the δ¹³C of naphthalene increased from -25.2‰ to -23.9‰ (enrichment of 1.3‰ ± 0.3‰) after a 95% of naphthalene biodegradation. For the unamended microcosms, a slight enrichment on δ¹³C napththalene was detected, from -25.2‰ to -24.5‰ (enrichment of 0.7‰ ± 0.3‰) after a biodegradation of approximately 65% of naphthalene (after 288 days). For sulfate reducing microcosms, no significant changes were detected on the δ¹³C during naphthalene biodegradation.