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 (7th to 9th grade; 9th to 11th grade) 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. (Contains 1 figure and 2 tables.)