The aim of the present study was to further examine the impact over time of single‐sex and coeducational school environments on girls’ motivation in language arts and mathematics. Two cohorts comprising 340 girls from eight coeducational and two single‐sex schools were followed during a period of three academic years in a longitudinal research scheme. Data were collected with a self‐reported questionnaire including several scales: parental and teachers’ support, competence beliefs, utility‐value and achievement goals. In general, mixed‐design repeated measures analyses of (...) variance indicated no effect of the environment or of the interaction between environment and time of measurement. Significant time effects on several variables indicated a general decline of achievement motivation over time. Consequently, the multiplication of non‐mixed high schools, as proposed by some, would constitute an expensive and inefficient social policy, as far as girls’ motivation is concerned. (shrink)