a sensor of force or viscosity or other attributes of a fluid comprises a mechanical resonator including an element of which the stiffness at least partially determines a modal shape of the resonance of the resonator and means for measuring a variation of a measure of the resonance as the stiffness of said element changes . the resonator may comprise two beams connected at or near one end by a yoke which provides a clamped condition of the resonator at said one end and connected at or near another end by said element .