The increase in the variety and capability of mobile communications devices carried by people today has made it possible to envision a new class of networks called Pocket Switched Networks (PSNs). In a PSN, because the nodes are constantly moving and their communication abilities are limited, the design of networking protocols and applications is challenging. This paper presents a new communications scheme for PSN, called Osmosis, based on the biological phenomenon. We show how this scheme can be applied to file sharing. This scheme uses epidemic routing to perform file lookup and achieves controlled flooding for file transfer. The flooding requires very little state information, which is collected during the lookup. This paper analyzes the performance of Osmosis in simulation studies based on real mobility traces.