In this paper, we propose Mutualcast, a new delivery mechanism for content distribution in peer-to-peer (P2P) networks. Compared with prior one-to-many content distribution approaches, Mutualcast achieves full utilization of the upload bandwidths of the peer nodes, thereby maximizing the delivery throughput. Mutualcast splits the to-be-distributed content into many small blocks, so that the more resourceful nodes may redistribute more blocks, and the less resourceful nodes may redistribute fewer blocks. Each content block is assigned to a single node for distribution, which can be a content-requesting peer node, a non-content-requesting peer node, or even the source node. The throughput of the distribution is controlled by redistribution queues between the source and the peer nodes. Furthermore, Mutualcast can be reliable and synchronous. Thus, it can be applied to file/software downloading, media streaming, real-time audio/video conferencing, etc.