This study examines the role of interlanguage transfer in lexico-semantic development in adult native Japanese-speaking learners of English as a second language in reference to two questions: (1) what constrains the initial hypothesis the student makes about the new target language word? and (2) what constrains a restructuring process of a potentially non-normative hypothesis? A semantic transfer hypothesis that predicts language transfer to be both pervasive and persistent in the domain of second language lexico-semantics is proposed, and it is suggested that this process operates together with the specific exemplar condition (stating that in second language learning, the student picks up and uses certain specific exemplars of a word, phrase, or sentence) and the prototype condition interactively in a total process of lexico-semantic development in this population. Implications are also drawn about the developmental relationship of over- and under-extensions. (MSE)