This study analysed the effect of high schools' gender organization on Korean tenth-grade students' science achievements, and their attitudes towards science. The high schools involved included an all-male institution, an all-female institution, and a co-educational institution. Three schools, three principals, three science teachers, and 302 tenth-grade students from their respective school types responded to an initial survey, and eleven academically outstanding students were subsequently interviewed. Also, the students' pencil and paper tests--which included second semester midterms, and final exams containing their general science test scores--were collected from each school. The questionnaire responses and the transcribed interview sections were analysed using One-Way ANOVA, followed by Post Hoc analysis, constant comparison, and content analysis. Results indicated that the male and female students from the co-ed school had significantly higher science achievement and positive attitudes towards science. Interview transcripts of the selected students from the all-male, the all-female, and the co-ed schools confirmed the findings. Later discussion addresses the improvement of the Korean science curriculum, of learning environments, of the teachers' and students' roles in the classroom, and of education policies. (Contains 1 table.)