Book excerpt: Semilingualism is discussed as a sociolinguistic concept and is viewed as a mediating variable when the society reproduces the class structure and vocational structure of suppressed minorities. A Finnish group of immigrant children in Sweden is the minority considered. It was found that the longer the Finnish children were educated in Finnish, the better their academic achievement in courses taught in Swedish. It was also found that children given no instruction in their mother tongue gradually lost competence in Finnish, without gaining full native-level competence in Swedish. This produced a subtractive bilingualism or semibilingualism, which hampered students' academic achievement. It is hypothesized that depriving students who come from a socially subordinated culture of instruction in their native language before the age of 10 may interfere with their cognitive development. The findings strongly support the case for extended bilingual education, and are relevant to those concerned with equality of educational opportunity for minority language children. (SW)