The flow velocity of ice stream D, West Antarctica has been measured to vary by a factor of three over the course of a day. These fluctuations are measured at the grounding line as well as upstream of the grounding line in the ice plain of ice stream D. The diurnal velocity fluctations appear to be driven by the tide beneath the Ross Ice Shelf. These results suggest that there is significant, and heretofore poorly understood, influence of the ocean tide and of the ice shelf on the dynamics of ice stream flow.