This paper considers a plasma microinstability driven by a macroscopic shear in the flow velocity parallel to a uniform magnetic field. The linear electrostatic dispersion relation for a Vlasov plasma is used. The flow shear instability which results has zero frequency. The wave number at maximum growth rate is of the order of the reciprocal ion gyroradius, but there is appreciable growth at the reciprocal of the electron gyroradius. This instability may provide wave-particle viscous effects at the magnetopause or stream-stream interfaces in the solar wind.