A study investigated the use of content-based tasks as central and peripheral in second language instruction. Specifically, it examined the relative effects of such tasks that were either central to the syllabus, as sources of knowledge or skills in their own right, or peripheral, as occasional source of language practice. The study was conducted with three groups of students in summer immersion programs: 12 Japanese teenagers with 3 native English-speaking informants; 18 Japanese college students; and 17 adult and secondary-level native English-speaking students enrolled in a Japanese course. Data on language use patterns were gathered in bilingual dyadic exchanges between native English-speakers and Japanese counterparts. Central and peripheral content-based tasks were analyzed for knowledge structures and repair type. Results suggest that learners make a variety of knowledge available to each other during negotiation over content-central tasks, suggesting two-way bilingual education may provide a richer context for learning both language and content. Similarly, content-central tasks, which permit open access to content knowledge, provide richer, more diverse background and situationally relevant knowledge than would content-peripheral tasks. However, it is also concluded that planned, content-peripheral tasks can be useful in providing highly contextualized language use. (MSE)