The study explored the effects of two kindergarten program organization factors--length of school day and class size--on kindergartners' reading, math and general knowledge achievement at the end of the kindergarten year. Two waves of data were drawn from the Early Childhood Longitudinal Study, Kindergarten Class of 1998-99 (ECLS-K) with an analytic sample of 15,575 children. A slight positive relationship was found between small class size and children's achievement in reading and math, particularly for children from minority and lower SES backgrounds. No relationship was linked with class size and general knowledge achievement. The relationship between full-day program and the three early academic skills was positive and statistically significant: almost all children made slightly higher gains in full-day programs compared with their counterparts in part-day programs. The findings suggest that policy makers may consider reducing very large class size and making full-day programs available to young children, particularly to poor minority children, at the same time aligning class size and length of kindergarten day with a child's characteristics and school curriculum.