We present a sensor fabricated with MEMS (Micro-Electro-Mechanical Systems) technology that upon immersion quickly measures fluid density and viscosity. The operational principal involves the influence of the fluid on the resonance frequency and quality factor of a vibrating plate oscillating normal to its plane. By performing measurements in liquids over a wide range of temperature (20 to 150 C) and pressure (0.1 to 75 MPa), we have demonstrated a maximum inaccuracy in our density and viscosity measurements of approximately +/- 1.5 % and +/- 10 % respectively, for fluids with densities between (0.6 to 1.5) g/cc and viscosities between (0.4 to 100) cP. Such measurements are required to determine the economic feasibility of recovering hydrocarbon from subterranean strata. There are numerous examples in the literature of sensors fabricated by the methods of MEMS that are claimed to measure both density and viscosity of fluids, but in most cases, the accuracy of such sensors is not been demonstrated in a wide range of fluids and moreover, their use in non-laboratory environments has not been proven.1,2,3 Here we show that it is possible to design and package a sensor that can function with high accuracy in extreme environments while providing useful information.