We examined the age of onset, over-time dynamics, and mechanisms underlying science achievement gaps in U.S. elementary and middle schools. To do so, we estimated multilevel growth models that included as predictors children's own general knowledge, reading and mathematics achievement, behavioral self-regulation, sociodemographics, other child- and family-level characteristics (e.g., parenting quality), and school-level characteristics (e.g., racial, ethnic, and economic composition; school academic climate). Analyses of a longitudinal sample of 7,757 children indicated large gaps in general knowledge already evident at kindergarten entry. Kindergarten general knowledge was the strongest predictor of first-grade general knowledge, which in turn was the strongest predictor of children's science achievement from third to eighth grade. Large science achievement gaps were evident when science achievement measures first became available in third grade. These gaps persisted until at least the end of eighth grade. Most or all of the observed science achievement gaps were explained by the study's many predictors. Efforts to address science achievement gaps in the United States likely require intensified early intervention efforts, particularly those delivered before the primary grades. If unaddressed, science achievement gaps emerge by kindergarten and continue until at least the end of eighth grade.