This study was made to discover whether or not teacher ideas and performance could be changed by a short English extension course on principles of composition, practical applications of linguistics, and various approaches to teaching slow learners. In 1965-66, three instructors spent 3 to 4 weeks in each of several schools in Illinois teaching their specialities to 66 junior and senior high school teachers. Before the extension course was taught, teachers completed questionnaires on their beliefs and teaching procedures. Each teacher was interviewed either 1 month or 12 months after completion of the course. The following data were gathered from the interviews: (1) 94% of the teachers claimed a change in teaching performance. (2) 61% claimed a change in thinking. (3) The kinds of changes reported by the teachers, who lacked consensus on the course's main ideas, did not coincide with the main ideas suggested by the instructors.(4) Very few teachers reported any constraints against utilizing the new ideas in their teaching situations. (5) Teachers interviewed after 1 month reported a 91% change in teacher performance; after 12 months they reported a 95% change. (6) An interviewer who was an instructor in the course found more change in teacher performance than did other interviewers. (Author/LH)