We examined the genetic and environmental contribution to the stability and instability of reading outcomes in early elementary school using a sample of 283 twin pairs drawn from the Western Reserve Reading Project. Twins were assessed across two measurement occasions. In Wave 1, children were either in kindergarten or first grade. Wave 2 assessments were conducted one year later. Results suggested substantial genetic stability across measurement occasions. Additionally, shared environmental influences also accounted for stability, particularly for variables more closely tied to direct instruction such as phonological awareness, letter knowledge, and word knowledge. There was also evidence for independent genetic and shared environmental effects, suggesting that new sources of variance may emerge as the demands of school change and children begin to acquire early reading skills.