The granular flow through an open silo is investigated experimentally. A mechanism based both on the dilation of the granular medium and an interaction with the interstitial gas causes the flow to stop at regular intervals. The experiments are carried out at different surrounding pressuresP0,and it is found that the intermittent flow becomes continuous at sufficiently lowP0,showing that the intermittency is linked to the interaction the gas. The scaling of the average flow rate with particle size further supports our view of the gas-grain interaction. ©1998 American Institute of Physics.