Many kindergarten teachers use ability groups to differentiate instruction in reading and math. Ability group placement should depend primarily on student achievement, but critics charge that placement is biased by socioeconomic status (SES), gender, and race/ethnicity. We predict group placement in the Early Childhood Longitudinal Study of the Kindergarten class of 2010-11, using linear and ordinal regression models with classroom fixed effects. The best predictors of group placement are test scores, but girls, high-SES students, and Asian Americans receive higher placements than test scores alone would predict. One third of students move groups during kindergarten, and high-SES students move up more than score gains would predict. Although group placement depends mainly on test scores, there are signs of bias.