This study was undertaken in order to investigate the hypothesis that students who participated in extended-day kindergarten sessions would show more significant gains in achievement on the California Achievement Test (CAT) than those students who participated in half-day sessions. Sixty-six students attending extended-day kindergarten sessions (experimental group) and 55 students attending half-day kindergarten sessions (control group) participated in the study. Subjects, after experiencing one year in either program, were administered the CAT and their mean scores were compared in order to test the hypothesis. Analysis of the mean scores indicated that the experimental group overscored the control group in both reading and math. These results, although not statistically significant, provided support for the study's hypothesis. A review of related research, comprising approximately two-thirds of the document, is provided following a brief discussion of the study's implications. A list of each subject's raw scores in both reading and math are appended. (Author/MP)