This study examined Taiwanese ESL learners'' perception and production of English interdental fricatives (IFs) with respect to three variables: perception errors, production errors, and three ranges of length of residence (LOR) in the US (LOR < 1 year; LOR= 3-4 years; LOR ? 7 years). Through the investigation, several relationships among perception, production errors, and LOR were revealed. In addition, the types and patterns of the pronunciation errors as well as the phonological processes involved in the mispronunciations were investigated. From the overall findings, ESL teachers are encouraged to apply multiple teaching tools and to provide feedback with effective correction techniques to learners in order to adjust and overcome their pronunciation errors.