Recently, multichannel protocols have been proposed and shown to significantly improve the aggregate throughput compared to single channel protocols. However, some existing proposals still suffer from low channel utilization or unfair bandwidth allocation between different flows in a multihop wireless network. In this paper, we design an asynchronous multichannel MAC called the cooperative multichannel MAC (COMMAC) that allows nodes to identify useful data channels using the channel usage information collected by their neighbors. Channel usage information includes primary users' activities. It also utilizes a new control message to prevent sending nodes from doing unnecessary backoffs. Via simulations, we compare our designed COMMAC with the AMCP scheme and show that our scheme can achieve 35% more throughput in multihop adhoc networks.