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This letter describes a fluidic microstrip bandstop filter with transmission properties that change in discrete states. The filter consists of a liquid metal alloy - eutectic gallium indium (EGaIn) - as the conductive component in microfluidic channels. The fluidity of EGaIn allows the open stub resonator of the filter to change its length by flowing in response to an applied pressure. A series of posts in the channel defines the length of the stub filled by the metal and dictates the pressure needed for the liquid metal to flow and thereby extend the stub length. The frequency response of the filter changes in response to the changes in the length of the resonator stub. This approach is a simple method for creating tunable filters and impedance matching sections using soft materials that change dimensions in response to pressure.