This paper presents an investigation of a tunable bandstop filter (BSF) using slotted ground structure. A kind of open loop slot resonator is used for the bandstop response and its characteristics according to various dimensions are considered to decide a proper shape for the final design. A barium-strontium-titanate (BST) varactor chip is deployed to tune each resonator and its location is determined by electromagnetic simulation for an optimum frequency tuning. A three-pole tunable BSF of this type is measured at the centre frequency from 4.5 to 5.5GHz and its rejection is more than 20dB at the stop band while the insertion loss at the pass band is less than 1.5dB. © The Institution of Engineering and Technology 2009.