A urea biosensor has been prepared by covalent binding of urease directly to the surface of an ammonium-sensitive field effect transistor (FET). The NH4+-sensitive membrane is based on a zeolite-incorporated polymeric membrane biosensor (clinoptilolite). The sensitivity of ammonium detection is sub-nernstian (32 mV/pNH4+) but the ISFET presents a high selectivity, which is interesting for measurements in biological media. The grafting of urease to the NH4+-sensitive membrane was performed by cross-linking with glutaraldehyde. The sensitivity of the urea ENFET is 15 mV/purea and this remains stable over 15 days with a detection limit of 3×10−5 M. Finally, in order to test feasibility of the urea biosensor for environmental applications, the remaining activity of the urease was determined after exposure to enzyme inhibiting heavy metals ions such as Hg(II). Using these urea biosensors, a detection limit of less than 5×10−8 M was obtained for Hg(II).