In this study, we investigated how gender and implicit theories of mathematics and science abilities interacted to predict mathematics and science achievements in a sample of elementary (primary) school students from the United States. Study participants came from six elementary schools (Grades 2 to 5) and were a part of a larger sample that participated in the Gifted Education in Mathematics and Science (GEMS) Project. We observed that, overall, boys had higher mathematics and science achievements than girls. Such gender differences were particularly significant among the 3rd and 4th graders. Moreover, there was a novel and yet consistent pattern of gender x implicit theories interaction effect on the achievements of the 3rd graders, such that boys had higher mathematics and science achievements than girls only when they endorsed strong incremental theories of mathematics and science abilities, but not when they endorsed weak incremental theories. The results demonstrate the importance of paying attention to early gender differences in mathematics and science achievements, particularly at transitional grades. The study also stimulates further investigations on how implicit theories of ability moderate such gender differences.