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We have proposed a scheme for supporting priority-based QoS in mobile ad hoc networks by classifying the traffic flows in the network into different priority classes, and giving different treatment to the flow-rates belonging to different classes. We have adopted a control-theoretic approach to adaptively control the low-priority flows so as to maintain the high priority flow-rates at their desired level, thus guaranteeing QoS to high-priority flow. At the same time, our objective is to adaptively maximize low priority flows while maintaining high priority flows at a desired level so that full utilization of wireless medium can be achieved through adaptive rate control. To provide this desired service guarantee to high priority flows, we need a distributed flow-control algorithm. Here, the low priority flows, causing interference to a high priority flow, detect and measure high priority flow-rate at each node on their routes and consequently adjust their flow-rates using a feedback control mechanism to maintain the high priority flow at its desired level. This detection and measurement is done at MAC layer of each node participating in routing from source to destination. We have proposed this protocol with a very nominal overhead using omni-directional antenna and modified the scheme to show the overall improvement in throughput using directional antenna. The performance has been evaluated using QualNet network simulator and the results indicate the effectiveness of our scheme.