The vocabulary development in the first 11 weeks of English acquisition by a 5-year-old Japanese girl was studied. The girl and her mother (a linguistic researcher) arrived in the United States at the start of the study. Lexical data from a language diary kept by the mother and from adult and peer sessions were pooled to produce a chronological corpus of vocabulary acquisition. The words in the chronological vocabulary corpus were classified according to semantic class. Lexical acquisition proceeded more rapidly than in first language development, with 75 words acquired by 7 weeks and 171 by 11 weeks. General nominals constituted 48% and multiword gestalt type lexical items constituted 12% of the total lexicon. Verbs, modifiers, and pronouns were more frequent than typically found in first language lexicons. Only 15% of the first 75 words and 11% of the total lexicon of 171 words were ever overextended. Overinclusive categorizing was less prominent and overextensions expressing presyntactic relations were relatively more prominent than in first language learning. It is concluded that vocabulary acquisition in second language development differs in a number of ways from first language lexical development. (Author/RW)