This study examines L1-L2 interaction in semantic categorization in early and late L2 learners. Word categories that overlapped but were not identical in Arabic and English were tested. Words always showed a "wider" range of application in one language, "narrower" in the other. Three types of categories--"classical", "radial", and "homophones"--were examined. Monolingual Arabic, monolingual English, early bilingual, and late bilingual speakers were tested for their understanding of the Arabic or English words. Early bilinguals' semantic structure is affected in both directions, late bilinguals' only in the direction of L1 to L2. Classical categories were most vulnerable to inter-language influence, whereas homophones were least vulnerable. The discussion addresses a developmental model of semantic interaction in early and late bilingual learners.