Alkaline phosphatase conductometric biosensors consisting of interdigitated gold electrodes and enzyme membranes have been used for assessment of heavy-metal ions in water. These analytes act as enzyme inhibitors. Enzyme residual activity has been measured in Tris-nitrate buffer without metal preincubation in the presence of Mg2+ ions as activator. The results indicate that the toxicity of the various metals tested toward immobilized phosphatase is ranged as follows: Cd2+ > Co2+ > Zn2+ > Ni2+ > Pb2+. Detection limits were about 0.5 ppm for Cd2+, 2 ppm for both Zn2+ and Co2+, 5 ppm for Ni2+ and 40 ppm for lead ions. In addition, the responses during 10 h were stable (RSD 4%) and a drift of about 7% per day was observed. The storage stability in buffer solution at 4 °C remained stable for more than one month.