A study was designed to investigate the nature of language proficiency and its cross-lingual dimensions. The focus of the study was on the interdependence hypothesis, that older immigrant students whose first language (L1) cognitive/academic proficiency is better developed on arrival in Canada will acquire English cognitive/academic skills more rapidly than younger immigrant students. The sample was comprised of Japanese children attending grades 2, 3, 5, and 7 of the School of Supplementary Japanese Studies in Toronto and of 45 recently arrived Vietnamese students between the ages of 9 and 17 years. Group and individual testing was done in both native language and English. Results are described separately for Japanese and Vietnamese studies. Data analyses supported the hypothesis that L1 cognitive/academic proficiency would account for a highly significant proportion of variance in second language (L2) proficiency. The data suggest that younger immigrant children tend to replace L1 as they acquire L2, whereas older children tend to add L2. With regard to the nature of language proficiency, findings suggest that L1 and L2 interactional style are interdependent because both are manifestations of personality attributes of the individual and of the same underlying cognitive/proficiency. (AMH)