Differentiation among students according to ability has been a topic of interest in educational systems all over the world for a long period of time. This study focuses on the extent of ability grouping in Swedish upper secondary schools, using a total population survey that covers all upper secondary schools. Previous research on the effects of ability grouping on students and groups of students gives a quite clear picture and the relation between ability grouping and issues of inequity and undemocratic values is an often-raised issue, where low-achieving students are especially adversely affected. Results from this study show that many (43%) upper secondary schools use ability grouping as a way to differentiate students in educational settings. It is found that it is more frequently used within the foundation subjects and especially in mathematics. No particular group of schools reports using ability grouping at a significantly higher rate, but the schools that reported using it to a very large extent differ on many school-level variables. These schools are focused on specifically. The extent of ability grouping is discussed in relation to previous research and issues of equity.