This paper explores the possibility that, contrary to the findings of past studies, the L1 and L2 mental lexicon may in fact be structurally similar, with depth of individual word knowledge determining a given word's degree of integration into the mental lexicon. The paper begins by reviewing the body of evidence relevant to the research question, and then presents the design and results of an investigation comparing nonnative and native speaker patterns of responses in light of depth of word knowledge scores. In discussing the results of the study, a tentative model for the process by which words are integrated into the mental lexicon is proposed, and the long-standing belief that a shift from predominantly syntagmatic to predominantly paradigmatic responses is indicative of lexical development is challenged.