Utilizing a diverse sample of 356 four-year-old children attending Head Start, this study examined the degree to which behavioral aspects of school readiness, including classroom participation, prosocial behavior, and aggression control were related to direct assessments of child cognitive readiness (academic knowledge, executive function skills) at the start of the prekindergarten year. Classroom participation and prosocial behavior each accounted for unique variance in cognitive readiness. Aggressive behavior, in contrast, was not correlated with academic knowledge, and was associated with low levels of executive function skills. In multiple regressions, aggressive behavior paradoxically enhanced the prediction of child cognitive readiness. Profile analyses strengthened the conclusion that the promotion of competencies associated with classroom participation and prosocial behavior may be particularly critical to cognitive readiness in prekindergarten. Implications are discussed for developmental models of school readiness and preschool classroom practice.