Asubmersiblemicrobial fuel cell (SBMFC) was developed as a biosensor forin situand real time monitoring of dissolvedoxygen (DO) in environmental waters. Domestic wastewater was utilized as sole fuel for powering the sensor. The sensor performance was firstly examined with tap water at varying DO levels. With an external resistance of 1000 Ω, the current density produced by the sensor (5.6±0.5~462.2±0.5 mA/m2) increased linearly with DO level up to 8.8±0.3 mg/L (regression coefficient, R2=0.9912), while the maximum response time for each measurement was less than 4 min. The current density showed different response to DO levels when different external resistances were applied, but a linear relationship was always observed. Investigation of the sensor performance at different substrate concentrations indicates that the organic matter contained in the domestic wastewater was sufficient to power the sensing activities. The sensor ability was further explored under different environmental conditions (e.g., pH, temperature, conductivity, alternative electron acceptor), and the results indicated that a calibration would be required before field application. Lastly, the sensor was tested with different environmental watersand the results showed no significant difference (p>0.05) with that measured by DO meter. The simple, compact SBMFC sensor showed promising potential for direct, inexpensive andrapid DO monitoringin various environmental waters.