Wireless sensor nodes lack hardware support for tamper- resistance and are often deployed in unattended environ- ments, thus leaving them vulnerable to capture and com- promise by an adversary. In a node replication attack, an adversary uses the credentials of a compromised node to surreptitiously introduce replicas of that node into the net- work. These replicas are then used to launch a variety of attacks that subvert the goal of the sensor application, and the operation of the underlying protocols. We present a novel distributed approach called Localized Multicast for detecting node replication attacks. We evaluate the perfor- mance and security of our approach both theoretically and via simulation. Our results show that Localized Multicast is more efficient than previous distributed approaches in terms of communication and memory costs. Further, in our ap- proach, the probability of detecting node replicas is much higher than that achieved in previous distributed protocols.