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We present an 18-MHz capacitive micromachined ultrasonic transducer (CMUT), used as a chemical sensor for detection of a common simulant for chemical weapons, dimethyl methylphosphonate (DMMP), in air. CMUTs are attractive for chemical sensor applications because of their unprecedented mass sensitivity per membrane area, low motional impedance, and high quality factor compared to other flexural-mode resonators. The device is composed of 1000 individual cells operating in parallel, which provides a robust operation with low false alarm rate and low motional impedance comparable to that of quartz crystal resonators. We designed a CMUT-based oscillator with a phase noise of -84.8 dBc/Hz and -136.6 dBc/Hz at offset frequencies of 1 kHz and 1 MHz, respectively. The oscillator exhibits an Allan deviation of 0.6 Hz with a one-sigma error of 0.037 Hz, which translates to a theoretical limit of mass detection of 16.2 ag. The 18-MHz CMUT is functionalized with a 50-nm thick polymer layer, polyisobutylene (PIB). The described CMUT sensor demonstrates a volume sensitivity of 37.38 ppb/Hz to DMMP.