The language learner is activated by exposure to primary linguistic data in the target language, categorizes that data and deduces from it a system of rules or hypotheses. When the language acquisition process is successful, as is virtually always the case in first language acquisition, the learner's rule system corresponds to that of the speech community at large. This is not the case of the second language learner. Adults attempting to learn a second language end up with a rule system which differs to one degree or another from the grammar of the target language. The learner's system of sentence-generating rules, which is very different from the grammar of his native language but not identical to the full grammar of the target language community, is called "interlanguage." When children are learning their native language, they are not exposed to truly random linguistic data; adults attempt to control and simplify their syntax when addressing them. The present study shows that a similar phenomenon occurs in second language learning. Native speakers simplify their language when talking to non-native speakers. Teachers, too, simplify their language according to the students' competence. The syntax of the oral classroom language of eight ESL (English as a second language) teacher-trainees was examined. The analysis of the data revealed that the teachers' classroom language was a potential factor in shaping the development of interlanguage. (Author/CFM)