The use of a series piezoelectric crystal (SPC) device for the preparation of biosensors has been investigated. These biosensors are fabricated by mounting the immobilized urease layer or the jack bean tissue slices on the probe surfaces of the SPC ammonia sensor. The urease sensor exhibits favourable frequency response to 1 x 10−5-3x10−3M urea with a response time of ca.3 min. Dynamic range, reproducibility, response time and selectivity of the sensors are discussed. Results show the SPC is an attractive alternative internal sensing element to develop biosensing devices. The urease sensor has been used successfully for the determination of urea in serum samples; the results are comparable to those from a spectrophotometric method.