While morphological awareness has received much attention to date, little is understood about how morphological awareness develops within bilingual children learning typologically different languages. Therefore, we investigated children's knowledge of inflections and derivations in Japanese and English, and also asked whether morphological awareness in one language predicted morphological awareness in the other. To that end, 24 Japanese learners of L2 English (ESL) and 21 English learners of Japanese as a heritage language (JHL) were recruited and participated in a range of tasks assessing both vocabulary and morphological knowledge. Cross-linguistic contributions of morphological awareness were identified in both directions (Japanese [if and only if] English), after controlling for age, IQ, and vocabulary knowledge. This bidirectional transfer was, however, identified only in the ESL group. The group-specific and reciprocal transfer observed is discussed in terms of morphological complexities and relative competence in each language. The potential role of different types of L2 instruction in morphological development is also discussed.