This study was designed to experimentally examine how supplemental vocabulary instruction provided in either whole-group or small-group settings influences low-income preschoolers' word knowledge and conceptual development. Using a within-subject design, 108 preschool children from 12 Head Start classrooms participated in an 8-week intervention, which included four topics of targeted vocabulary instruction counterbalanced in either a whole-group or small-group configuration. Pre- and posttest measures examined children's outcomes in word learning and in conceptual and categorical knowledge. Our results indicated that group size did not appear to be a powerful mechanism for intensifying instruction. Although children gained significantly in word knowledge, concepts, and categories, they did so regardless of whether they were in small or whole groups. Implications for these findings, as well as limitations of the research and directions for future research, are discussed. (Contains 5 tables and 1 note.)