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In this letter, an elliptic low-pass filter (LPF) implemented in coplanar waveguide (CPW) technology by etching open complementary split ring resonators (OCSRRs) in the central strip is presented for the first time. The OCSRRs behave as series connected parallel resonant tanks, whereas metallic patches etched in the back side of the substrate provide the required shunt capacitance to achieve the elliptic function response. An order-5 elliptic LPF has been designed and fabricated to illustrate the possibilities of this new approach. The measured frequency response is in good agreement with the ideal elliptic function up to twice the cutoff frequency (fc=1& GHz). Since OCSRRs are electrically small resonators, filter dimensions are also small (device length is 2.5 cm, namely 0.13 lambda , where lambda is the guided wavelength at fc). This work is illustrative of the possibilities of OCSRRs for the design of compact planar filters and other microwave components.