This work describes the development and testing of a sensitive and selective potentiometric nitrate microsensor based on doped polypyrrole films. Utilizing 6–7 µm carbon fibers as a substrate for pyrrole electropolymerization allowed fabrication of flexible, miniature and inexpensive sensors for in situ monitoring of nitrate. The sensors have a rapid response (several seconds) and in their characteristics are competitive with expensive commercial nitrate ion selective electrodes (ISE), exhibiting Nernstian behavior (slopes 54 ± 1 mV per log cycle of nitrate concentration (n=8), at T=22°C), a linear response to nitrate concentrations spanning three orders of magnitude (0.1–10⁻⁴ M or 6200–6.2 ppm of NO₃⁻), and a detection limit of (3 ± 1) × 10⁻⁵ M (1.25–2.5 ppm). After a 2-month-period, the response was unchanged, and after 4.5 months, one version of the electrode continued to exhibit significant sensitivity to nitrate. Several polypyrrole nitrate microsensors were embedded sequentially downstream from a point source of nitrate solution in an intermediate scale physical groundwater model as a test of their performance under simulated environmental conditions. The microsensors responded appropriately to the approaching nitrate solution front, demonstrating dispersion and attenuation of the nitrate concentrations that increased with distance from the source.