This study examines bilingual vocabulary knowledge in relation to arrival age among first language (L1) Japanese students attending hoshuukoo (i.e., supplementary academic schools for Japanese-speaking children) in the United States. It also examines the relationship between L1 Japanese and English as a second language (L2), as motivated by Cummins's (1979, 1991) notion of linguistic interdependence. One hundred and twenty-two high school students ages 15–18 from eight hoshuukoo took Japanese and English vocabulary tests designed by Ono (1989). Students who came to the United States by age 9 or younger were three grades behind in L1 Japanese and were either ahead of or at their U.S. grade level in English. In contrast, those who arrived at age 10 or older were just one grade behind in Japanese and were two to five years behind in English. High vocabulary knowledge in one language was associated with low knowledge in the other, and the negative correlation between L1 and L2 became statistically nonsignificant when arrival age was controlled. Consequently, arrival age remains an important factor in accounting for hoshuukoo students' bilingual vocabulary learning, and the notion of linguistic interdependence must be reexamined in factors in addition to vocabulary knowledge.