This letter presents the design of a compact bandstop filter (BSF) operating at two frequencies. The proposed BSF consists of open-ended stepped impedance resonators (OSIR) and an end-shorted parallel-coupled microstrip line (E-PCML). The OSIRs are used to achieve the impedance-controlled stopband positions. The wide BSF bandwidths are achieved through enhanced coupling of the E-PCML. Explicit design guidelines are derived using a lossless transmission line model. To validate theoretical predictions, a prototype dual-band BSF operating at 900 MHz and 2,100 MHz with fractional bandwidths of 72% and 36%, respectively, is implemented in microstrip.