Regardless of individual differences at kindergarten entry, schools have a mission to promote reading achievement for all students. Within-class ability grouping is an instructional strategy that has received attention for its potential benefits to students. The authors assessed the effects of within-class ability grouping on kindergarten reading growth by using data from the Early Childhood Longitudinal Study-Kindergarten Cohort. Results revealed that the frequency with which teachers used ability groups was positively associated with mean school gain in reading, suggesting that early literacy and reading improvement in kindergarten may be facilitated by the use of ability groups in reading. (Contains 5 tables and 1 note.)