A microbial amperometric sensor based on the yeast Arxula adeninivorans was tested to determine its suitability for measuring biochemical oxygen demand (BOD) in salt water. The viability of cells immobilized onto the sensor membrane was hardly inÑuenced up to 10% (w/v) NaCl in the sample, although the solubility of oxygen was a †ected. NaCl concentrations higher than 10% (w/v) caused a marked decrease in the oxygen solubility and deactivated the sensor. This outcome depended on the substrates used, e.g., alanine-, galactose-and acetic acid-sensor signals were inÑuenced by any salt concentration whereas glucose-, glycerol-, maltose-and arginine-sensor signals were inÑuenced only by higher salt concentrations. Sensor signals from yeast extract as well as glucose correlated with the quantity of these substances and with the salt concentration contained in the water. This correlation was linear up to 10% (w/v) NaCl and 0É125% (w/v) yeast extract or up to 10% (w/v) NaCl and 0É125% (w/v) glucose in the sample. The sensor signals are therefore inÑuenced only by NaCl-determined solubility of oxygen and not by the physiological parameters of the immobilized cells. However, an increase of yeast extract-or glucose-concentrations in the presence of NaCl caused physiological e †ects on the sensor cells.