A study of the differences in achievement among Ottawa's grade 9 and 10 students taught in English and French in three subject areas (mathematics, geography, and history) in which suitable comparative measures were available had as subjects the students in 73 classes. The students in French-language classes were administered the test twice, once in English and once in French; half took the English test first, and half took the French test first. The instruments used were tests developed by the Carleton Board of Education. An attempt was made to correct for aptitude differences between groups and to examine item analysis data for group differences. Information concerning student use of French outside the classroom, difficulties related to a change in language of instruction, advantages of French-language instruction, and the subjects they preferred to be taught in French was also gathered from students and teachers. The test results suggest some differences in achievement between the subject matter areas, with the students taught mathematics in French performing better than in English, but those taught history and geography in French performing as well as those taught in English. Some problems were found with the marking system in which students have grades lowered for errors in French, i.e., in which language across the curriculum is emphasized. Limited opportunities to speak French were identified as an area for improvement, and both students and teachers mentioned limited student ability for both personal and technical expression as a problem. (MSE)