We have observed pumping of water induced by 4.7 MHz ultrasonic Lamb waves traveling in a 4-μm-thick composite membrane of silicon nitride and piezoelectric zinc oxide. The observed pumping speed is proportional to the square of the wave amplitude; the speed was 100 μm/s for a rf drive voltage of 8 V and a 6.5 nm wave amplitude. A nonlinear model based on acoustic streaming theory predicts velocities in good agreement with experiment.