This study examined differences between 326 students enrolled in a full-day kindergarten program and 311 students in a half-day program. Data were obtained from six measures of academic achievement from the California Achievement Tests, administered in the spring. Results indicated no significant differences between the two groups on four measures of academic achievement--visual recognition, sound recognition, vocabulary, and language expression. Significant differences were found on two scores, comprehension and mathematics concepts and applications. Further analysis determined that the difference in comprehension scores was due to girls in the half-day program scoring higher than boys in the full-day program and could not be attributed to differences in the programs. The difference in mathematics concepts and applications scores was due to boys in the full-day program significantly outscoring boys in the half-day program. (Includes 23 references.) (MDM)