An ammonia chemiresistor sensor is demonstrated on the basis of a poly(3-hexylthiophene) film oxidized by nitrosonium hexafluorophosphate. The sensor gave a good response to ammonia, but it was insensitive to a variety of organic vapors. The detection limit was estimated to be ca. 0.22 ppm. The redox reaction between ammonia and bipolarons was attributed to the sensor signals.