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In this paper we describe the design, implementation and performance evaluation of a low-power spectrum agile medium access control protocol for wireless sensor networks. With the ever increasing popularity of wireless embedded devices and networks, spectrum is getting congested, which in turn leads to performance degradation. Since protocols are designed in isolation of each other without appropriate consideration for potential interferences and mechanisms for symbiotic coexistence, they fail to achieve the desired performance characteristics in realistic interfering environments. The performance degradation is more significant for low-power embedded networks as they remain handicapped when competing with less resource constrained networks. We design a protocol that allows sensor nodes to dynamically select an interference minimal channel for data communication. It does not pose any synchronization restrictions on the nodes and effectively handles the dynamics of the network such as new nodes joining and old nodes leaving the network. We describe the various energy efficient spectrum sensing features of the protocol on which the dynamic channel selection is based. Our experiments suggest that even in highly crowded spectrum and environments with random interferences, sensor nodes are able to communicate in a reliable and energy efficient manner.