Speech samples were taken every two weeks for a period of 40 weeks from a five-year-old Japanese girl learning English as a second language through her environment. The presence or absence of some grammatical morphemes in linguistic or nonlinguistic obligatory context was scored; using Brown's (1973) criterion for morpheme acquisition, a rank order of these morphemes was determined. The order obtained was different from those of native English-speaking children. Possible determinants for this rank order are hypothesized in terms of (1) presence/nonpresence of that semantic notion expressed by those individual English morphemes in Japanese, (2) a principle of simplicity, and (3) phonological interference. (Author)