This study investigated second language (L2) lexical development in the spontaneous speech of six adult, L2 English learners in a 1-year longitudinal study. One important aspect of lexical development is lexical organization and depth of knowledge. Hypernymic relations, the hierarchical relationships among related words that vary in relation to their semantic specificity (e.g., Golden Retriever vs. dog vs. animal), are an important indicator of both lexical organization and depth of knowledge. Thus, this study used hypernymy values from the WordNet database and a lexical diversity measure to analyze lexical development. Statistical analyses in this study indicated that both hypernymic relations and lexical diversity in L2 learners increase over time. Additionally, lexical diversity and hypernymic values correlated significantly, suggesting that as learners' lexicons grow, learners have access to a wider range of hypernymy levels. These findings are discussed in relation to developing abstractness in language, extending hypernymic knowledge, and the growth of lexical networks.