The purpose of this study was to evaluate the relationship between physical fitness and academic achievement. To do so, scores on the FITNESSGRAM®, a physical fitness test, were compared to reading and mathematics scores on the Stanford Achievement Test 9th edition, a standardized norm-referenced achievement test. Subjects were all 5th, 7th, and 9th grade California school children enrolled in public school in 2002 for whom there was complete data on both the physical fitness and academic achievement tests. The sample size was 884,715 students. Results indicate a consistent positive relationship between overall fitness and academic achievement. That is, as overall fitness scores improved, mean achievement scores also improved. This relationship between fitness and achievement appeared to be stronger for females than males and stronger for higher socio-economic status (SES) than lower SES students. Results should be interpreted with caution. It cannot be inferred from these data that physical fitness causes academic achievement to improve. It is more likely that physical and mental processes influence each other in ways that are still being understood.