In today?s Internet a user can be adversely affected by other users that overload the router. To address this problem, routers need to provide flow isolation. In this paper, we present MuxQ, a new queue management mechanism that provides a high degree of isolation without using per-flow queuing. MuxQ protects the multiplexing function of the router buffer by progressively controlling the allocation of buffer space in a FIFO queue. The allocation decision is based on state information of only a limited number of flows: the flows that do currently have packets in the queue. We evaluate MuxQ by simulation and show that it performs vastly better than the classical Drop-Tail. By using a very simple algorithm MuxQ provides reasonable flow isolation.