An experimental investigation was conducted with an oscillatory flow water tunnel to elucidate the mechanism of sediment entrainment by periodic fluid motion past rippled beds. The instrumentation used included an optical-type sediment concentration analyzer, a hot-wire anemometer, and an IBM 1801 Data Acquisition and Control System. The signal-averaging technique used to analyze the data permitted decomposition of the concentration and velocity signals into mean, periodic, and random components. The process of sediment entrainment and suspension is explained in terms of flow separation from the ripple crest together with the flow reversal. Simultaneous measurement of temporal and spatial distributions of suspended sediment concentration. Simultaneous measurement of temporal and spatial distributions of suspended sediment relationship and vertical fluid velocity in the region just above the bed showed that the conventional one-dimensional continuity relationship for steady uniform flow is inadequate; the horizontal gradient of streamwise sediment flux is comparable in importance to the vertical gradient of vertical flux of sediment.