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We present a resonant chemical sensor based on a capacitive micromachined ultrasonic transducer (CMUT) technology. Depending on the frequency of the devices (18 to 32 MHz), the mass sensitivity per unit area ranges from 73 to 130 zg/Hz/mum2. We functionalized the 18-MHz device with polyisobutylene (PIB) to detect dimethyl methylphosphonate (DMMP), a common simulant for the sarin nerve agent. Even with only a 50-nm thick coating layer, our sensor has a high volume sensitivity of 37 ppbv/Hz to DMMP in air. Taking advantage of multiple CMUT cells (100 to 2240), all resonating in parallel, the sensor achieves an equivalent volume resolution of 21 ppbv (parts per 109 by volume) to DMMP. In addition, 200 test cycles with DMMP applied over 26 hours revealed a zero false alarm rate and a 4.7% (3-sigma) variation of volume sensitivity to DMMP. By using principal component analysis (PCA), we successfully classified all analytes in 21 experiments, and we present the results of pattern recognition. This work demonstrates that CMUT has a great potential for the sensitive, reliable, and yet portable chemical sensing systems.