A fibre optic sensor for the detection of dissolved ammonia in concentrations below 100 µg L⁻¹ is presented. The sensing principle is based on an immobilized fluorescent pH indicator which is deprotonated by ammonia. The resulting ammonium ion is stabilized by a cation trap. The components were immobilized in a cellulose ester as matrix polymer. Microsensors prepared with this sensor chemistry showed a detection limit of 0.5 µg L⁻¹. The dynamic range of the sensor matches perfectly with the concentration levels (<25 µg L⁻¹) that are known to be toxic for organisms. The determination of ammonia is important, because of the toxicity of this compound to organisms. Microsensors are powerful tools to establish microprofiles of high spatial resolution with the least disturbance of the ecological system. Optical microsensors for ammonia can be easily fabricated using the novel sensor chemistry. The potential interference of trimethylamine was minimized using an 18-crown-6 ether derivative as the cation trap and by the permeability properties of the polymer matrix. Cross-sensitivities towards protons and alkali ions are prevented by a Teflon coating covering the ammonia sensitive layer. This makes the sensor suitable for measurements in fresh and salt water.