This work provides a solution to identify malicious nodes in wireless sensor networks through detection of malicious message transmissions in a network. A message transmission is considered suspicious if its signal strength is incompatible with its originator's geographical position. We provide protocols for detecting suspicious transmissions — and the consequent identi.cation of malicious nodes — and for disseminating this information in the network. We evaluate the detection rate and the efficiency of our solution along a number of parameters.