This study acoustically analyzed the production of single and geminate stops in Japanese by English-speaking children (N = 19) at three different grade levels in a Japanese immersion program. Results show that both their singletons and geminates were significantly longer than those of Japanese monolinguals and the bilinguals' immersion teachers, but all of the immersion groups have acquired the contrast between the two types of stop. This finding supports Flege's (1995) hypothesis that a phonetic category established for second language sounds by a bilingual might differ from that of a monolingual. Additionally, 52 native speakers of Japanese rated the contrast between the two stops produced by all of the bilingual children and a subset of the monolingual children. The accent ratings suggest that the contrast made by the immersion children was not nativelike despite some individual differences in their performance and that there was no statistical difference in accent ratings across the grade levels. The degree of the contrast correlated fairly highly with the closure duration ratio of geminates to singletons.