The knowledge base on various forms of structuring students' learning by ability grouping is more robust than that on teachers' instructional practices being implemented within these groupings. The present study examines if student-reported mathematics teachers' instructional practices vary among schools with different degrees of implementing between-class ability grouping and if between-class ability grouping moderates the relationship between these practices and students' mathematics achievement. International data from 281,591 fifteen-year-old students and 11,765 principals were analysed. One-way ANOVA showed that students perceived greater implementation of teachers' instructional practices in schools with more between-class ability grouping. However, hierarchical linear modelling (HLM) results showed that between-class ability grouping did not moderate the relationship between teachers' instructional practices and students' mathematics achievement. These results imply that it is more important to examine the relationship between teachers' instructional practices and students' achievement in homogenous ability classes vis-à-vis that between ability grouping and students' achievement per se.