The environmental risk assessment of toxic chemicals in stream water requires the use of a low cost standardized toxicity bioassay. Here, a biosensor for detection of toxic chemicals in stream water was studied using sulfur oxidizing bacteria (SOB) in continuous mode. The biosensor depends on the ability of SOB to oxidize sulfur particles under aerobic conditions to produce sulfuric acid. The reaction results in an increase in electrical conductivity (EC) and a decrease in pH. The biosensor is based on the inhibition of SOB in the presence of toxic chemicals by measuring changes in EC and pH. We found that the SOB biosensor can detect Cr(6+)at a low concentration (50 ppb) which is lower than many whole-cell biosensors. The effect of organic material in real stream water on SOB activity was studied. Due to the presence of mixotrophic SOB, we found that the presence of organic matter increases SOB activity which decreases the biosensor start up period. Low alkalinity (22 mg L(-1) CaCO(3)) increased effluent EC and decreased effluent pH which is optimal for biosensor operation. While at high alkalinity (820 mg L(-1) CaCO(3), the activity of SOB little decreased. We found that system can detect 50 ppb of Cr(6+) at low alkalinity (22 mg L(-1) CaCO(3)) in few hours while, complete inhibition was observed after 35 h of operation at high alkalinity (820 mg L(-1) CaCO(3)).