This article uses data from a research project exploring grouping practices based on 'ability' in classrooms for children aged 3-7 years in England to consider the relationship between teachers' views of ability and their ways of organising children. The widespread use of grouping with young children and the concomitant 'fixed-ability thinking' by teachers are discussed, alongside an exploration of how and why teachers object to grouping on this basis. Examples of teachers who were able to disrupt grouping practices based on 'ability' are described, allowing for a further discussion of the barriers to change for the majority of teachers. The article concludes that the relationship between teachers' beliefs about ability and their grouping practice is complex, as there can be both grouping without fixed-ability thinking and vice versa.