Results obtained from longitudinal studies suggest that individual differences in reading performance are relatively stable over time. However, the aetiology of this stability has not been previously explored. In the current study, the aetiology of longitudinal stability of reading performance between 7 and 12 years of age was assessed using data from adoptive (97 unrelated sibling pairs at age 7 and 73 pairs at age 12) and nonadoptive (106 related pairs at age 7 and 75 pairs at age 12) children tested in the Colorado Adoption Project. Results of a bivariate behavioural genetic analysis confirmed earlier findings of moderate genetic influence on individual differences in reading performance at both 7 and 12 years of age ( h2 = .49 and .37, respectively). Moreover, about 70% of the observed stability ( r = .61) between the two ages was due to common genetic influences. Of special interest, no new heritable or shared environmental variation was manifested at age 12, suggesting that the same genetic and shared environmental influences were operating at both ages. In contrast, nonshared environmental influences (e.g. instructional methods, teachers, peers, etc.) were responsible for change between 7 and 12 years of age, indicating the salience of such factors for the development of reading performance between middle childhood and adolescence.