Automatic target segmentation is critical to computerized dental imaging systems, which are designed to reduce human effort and error. We have developed an automatic algorithm that is capable of outlining an intra-oral reference bar and the tooth of interest. In particular, the algorithm first locates the reference bar using unique color and shape cues. The located reference bar provides an estimate for the tooth of interest in terms of both its scale and location. Next, the estimate is used to initialize a trained active shape model (ASM) consisting of the bar and the tooth. Finally, a search process is performed to find a match between the ASM and the local image structures. Experimental results have shown that our fully automatic algorithm provides accurate segmentation of both the reference bar and the tooth of interest, and it is insensitive to lighting, tooth color, and tooth-shape variations.