We demonstrate a method for high-bandwidth, high-sensitivity particle sensing and cell counting in a microfluidic system. Our approach employs a tuned radiofrequency probe, which forms the radiofrequency analog of a Coulter counter. By measuring the reflected rf power, this approach provides an unprecedented detection rate, with a theoretical bandwidth in excess of 10 MHz. Particle detection was performed in a continuous flow mode in a microfluidic channel, using 15 μm diameter polystyrene beads suspended in a sucrose-saline solution. We demonstrate 30 kHz counting rates and show high-resolution bead time-of-flight data, comprising the fastest electronic particle detection on-chip to date.