Although the speaking section of the TOEFL iBT test is used by many universities to determine if international teaching assistants (ITAs) have the oral proficiency necessary to be classroom instructors, relatively few studies have investigated the validity of using TOEFL iBT scores for ITA screening. The primary purpose of this study was to determine the effectiveness of using TOEFL iBT Speaking and Listening scores for ITAs in the context of an urban research university. This was accomplished by correlating the TOEFL iBT test scores with the scores obtained by prospective ITAs on local screening tests and student evaluations of ITAs' language and teaching competence and by using the TOEFL iBT test scores as predictors of ITA teaching performance as measured by student evaluations. Given that listening comprehension is an important aspect of instructional language competence, both TOEFL iBT Speaking and TOEFL iBT Listening scores were used as predictors. The development of ITAs' oral proficiency over a semester in an English-speaking environment was also examined by comparing their TOEFL iBT Speaking and Listening scores at the beginning and at the end of their first semester as ITAs. The results indicate that TOEFL iBT Speaking and Listening scores correlate moderately with the local screening exam, but more importantly, the results indicate that TOEFL iBT Listening might be a better predictor of teaching competence. Although TOEFL iBT Speaking scores did not correlate with or predict the various measures of teaching competence, TOEFL iBT Listening scores did. In addition, the study found that ITAs living and teaching in an English-speaking environment did make measurable gains in oral proficiency over the course of a 3-month period. Although these gains were relatively small, and the listening score gains were not statistically significant, this is the first study that has actually tried to measure the improvement in ITAs' oral proficiency while living and studying (and sometimes teaching) on campus. The importance of this study goes beyond the local context. This is the first study to examine the relationship between TOEFL iBT Speaking and Listening scores and instructional performance of ITAs, thus serving as a validation study of the use of the TOEFL iBT test for ITA placement purposes. The study also offers insights into ITAs' development of oral proficiency once they are in an English-speaking environment, which can serve as a basis for future ITA curriculum development.