This article reports a cross-national study of classroom environments in Australia and Indonesia. A modified version of the What Is Happening In this Class? (WIHIC) questionnaire was used simultaneously in these two countries to: 1) cross validate the modified WIHIC; 2) investigate differences between countries and sexes in perceptions of classroom environment; and 3) investigate associations between students' attitudes to science and their perceptions of classroom environment. The sample consisted of 1,161 students (594 students from 18 classes in Indonesia and 567 students from 18 classes in Australia). Principal components factor analysis with varimax rotation supported the validity of a revised structure for the WIHIC. Two-way MANOVA revealed some differences between countries and between sexes in students' perceptions of their classroom environments. Simple correlation and multiple regression analyses revealed generally positive associations between the classroom environment and student attitudes to science in both countries.