Few, if any, researchers would deny the existence of the developmental stages of language learning. However, there are questions about the applicability and the importance of the stages in pedagogy. Up to this point, these questions regarding the ESL (English as a second language) stages have never been addressed in a Japanese post-secondary educational context. This study is the only one of its kind to measure the developmental stages of a group of Japanese university students and to provide the learners with intensive instruction to see how much/if any changes are made to their interlanguage due to the instruction. As predicted by Pienemann's Teachability Hypothesis (1992), only the learners who had met the prerequisites for instruction could acquire the grammatical points which were instructed. The results of this study point to the need among TESOL (Teaching English as a Second or Other Language) instructors to teach students only slightly above their current language levels. (Contains 5 tables.)