A single-crystal silicon resonant bulk acoustic mass sensor with a measured resolution of 125 pg/cm2 is presented. The mass sensor comprises a micromachined silicon plate that is excited in the square-extensional bulk acoustic resonant mode at a frequency of 2.182 MHz, with a quality factor exceeding 106 . The mass sensor has a measured mass to frequency shift sensitivity of 132 Hz cm2 / g. The resonator element is embedded in a feedback loop of an electronic amplifier to implement an oscillator with a short term frequency stability of better than 7 ppb at an operating pressure of 3.8 mTorr.