The use of structured ability grouping is increasing in English primary schools and is regularly seen in primary mathematics classrooms. Ability is a normalised discourse with beliefs that some individuals are "born to do maths" permeating society and infiltrating school practices. In this article, observation and interview data illustrate the persistence of fixed-ability thinking, even in situations where explicit ability grouping practices are not used. The data analysis suggests a mismatch between mixed ability practices and fixed-ability thinking, and the article argues that change will be difficult.