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 influenced up to 10% (w/v) NaCl in the sample, although the solubility of oxygen was affected. 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 influenced by any salt concentration whereas glucose-, glycerol-, maltose- and arginine-sensor signals were influenced 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 0.125% (w/v) yeast extract or up to 10% (w/v) NaCl and 0.125% (w/v) glucose in the sample. The sensor signals are therefore influenced 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 effects on the sensor cells.