A rheometer which uses a piezoelectric ultrasonic composite oscillator has been developed to measure the viscoelastic properties,G’(&ohgr;) andG‘(&ohgr;), of polymer melts at ultrasonic frequencies. The storage and loss moduli of an ethylene–tetrafluoroethylene copolymer melt were measured at six frequencies in the range 0.25×106–1.28×106rad/s (between 40 and 200 kHz nominal frequency) and at a temperature of 280 °C. This ultrasonic instrument operates at strain amplitudes ranging from about 10−8to 10−3and is therefore ideally suited for linear viscoelastic property measurements.