Abstract A microbial cyanide sensor using Saccharomyces cerevisiae for monitoring a river water is described. This sensor is based on the inhibition of S. Cerevisiae's respiration by cyanide. This sensor is a reactor type flow system and composed of two oxygen electrodes and a reactor which contains S. cerevisiae immobilized beads. The S. cerevisiae's respiration activity is measured using the oxygen electrodes. The sensor showed a linear response in the range from 0 to 15 μM and maintained stable response for 9 days at ambient temperature. The sensor was optimized for the monitoring of river water and was applied to river water analysis.