The present study uses a parent-offspring adoption design to examine the dual roles of heritable and environmental influences on children's mathematics achievement. Linked sets (N = 195) of adopted children, adoptive parents, and birth parents each completed a measure of mathematics fluency (i.e., simple computational operations). Birth parent mathematics achievement and adoptive father mathematics achievement positively correlated with child achievement scores at age 7, whereas adoptive mother and adopted child mathematics achievement scores were not significantly associated with one another. Additionally, findings demonstrated no significant effects of gene-environment (GxE) interactions on child mathematics achievement at age 7. These results indicate that both heritable and rearing environmental factors contribute to children' mathematics achievement and identify unique influences of the paternal rearing environment on mathematics achievement in middle childhood. [This paper was published in "Developmental Science" v23 n6 2020.]