The authors analyzed the Early Childhood Longitudinal Study-Kindergarten Cohort (ECLS-K) national data set to investigate gender differences in ability group placement in American kindergartens. They found that in kindergarten, within-class ability grouping was widely used for reading instruction, with boys being underrepresented in high-achieving reading groups and overrepresented in low-achieving ones. Gender differences in reading group placement were consistent across classrooms and were explained by student-level characteristics. Boys' underrepresentation in high reading groups was explained by their lower reading skills at kindergarten entry, as measured by the reading test scores available in the ECLS-K. By contrast, boys' overrepresentation in low reading groups was only partially explained by their lower test scores. Compared with girls of similar social background and reading test scores, boys continued to have higher chances of placement into a low reading group. This remaining gender difference was explained by the lower teacher evaluations of boys' reading skills and approaches to learning. Boys' disadvantages in reading group placement at school entry raise concern over their further academic success. (Contains 3 tables, 1 figure and 3 notes.)