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Security is one of the most important challenges in mobile ad hoc networks (MANETs). In this context, gray hole attack is a kind of black hole attacks which are produced by malicious nodes in MANETs and has important impact on MANETs efficiency. This attack is produced as a result of extensive advertisements in response to path requests made on the part of the source node in MANET. So, an adversary nodes deceive normal nodes of the network and announce that they possess the shortest path for reaching the destination. Indeed, normal nodes register the malicious nodes as new members in the transmission and reception process; however, malicious nodes suddenly show negative behaviors and they remove a significant amount of the reached packets. In this paper, we proposed a new scheme to detect and isolate gray hole attacks in MANETs. The simulation results in NS2 indicates that the proposed scheme improves end-to-end delay, throughput and packet delivery rate in comparing with other methods.