The increasing use of wireless sensors networks in Supervisory Control and Data Acquisition systems (SCADA) raises the need of enforcing the security of this promising technology. Indeed, SCADA systems are used to manage critical installations that have hard security, reliability and real-time requirements. Consequently, in order to ensure Wireless Industrial Sensor Networks (WISN) security, Intrusion Detection Systems should be used as a second line of defense, in addition to sensor's embedded security mechanisms. In this paper, we present wIDS a multilayer specification-based Intrusion Detection System specially tailored for WISN. It has a two-level detection architecture and is based on a formal description of node's normal behavior.