A microsensor for urea is described which is based on immobilised urease on the tip of a 10 µm diameter ammonia gas electrode made from a polypyrrole film coated on to a platinum wire. The sensor responds rapidly, reaching a steady state within 20–40 s when the concentration of urea in solution is changed from 0.0001 to 0.05 M. The enzymatic response is current limiting. Responses in a wide range of substrate concentrations were achieved (0.001–0.05 M). High storage and operational stability (more than 32 d) have been recorded for the urea microsensor.