Purpose: Children from low--socioeconomic status families often perform poorly on standardized vocabulary assessments. The primary purpose of the study was to determine whether lexical diversity as measured by "D" (Malvern, Richards, Chipere, & Durán, 2004) serves as a valid measure of vocabulary in at-risk, low-income, predominantly African American kindergartners. Method: Kane's (1992) argument-based approach was used to validate "D." Six assumptions were examined. Kindergartners (N = 210) from a high-poverty, low-achievement region of the United States were recorded narrating a wordless picture book and assessed using the Expressive Vocabulary Test, Second Edition (Williams, 2007), and the Kaufman Test of Educational Achievement, Second Edition-Listening Comprehension subtest (Kaufman & Kaufman, 2004). Results: "D" was distributed normally and did not vary as a function of language sample length or child ethnicity. "D" was significantly but weakly related to the Expressive Vocabulary Test, Second Edition, indicating some distinction between D and the Expressive Vocabulary Test, Second Edition, scores. Further, "D" was only marginally related to the Kaufman Test of Educational Achievement, Second Edition-Listening Comprehension subtest. Conclusions: Although evidence was somewhat mixed, the study supported the view that "D" is a potentially valid measure of lexical diversity among low-income, predominantly African American kindergartners and could be a useful supplement to standardized vocabulary measures.