Abstract. This paper presents the audio based tracking system designed at FBK-irst laboratories for the CLEAR 2007 evaluation campaign. The tracker relies on the Global Coherence Field theory that has proved to efficiently deal with the foreseen scenarios. Particular emphasis is given to the post-processing of localization hypotheses which guarantees smooth speaker trajectories and is crucial for the overall performance of the system. The system is also equipped with a speech activity detector based on HMM. The performance delivered by the proposed tracker presents a considerable gain with respect to the previous evaluation.