Internationally and historically in the UK, considerable research has been undertaken regarding teachers' pedagogical practices when working with high or low ability classes. Latterly, in the UK, research has focused on pedagogical practices in mixed ability classes. This paper aims to compare secondary school teachers' pedagogical practices in mixed and ability grouped classes and explore whether these are influenced by the extent to which either practice was influenced by the type of grouping practices adopted in the school within which they worked. The sample comprised over 1500 teachers from 45 schools divided into three groups based on their ability grouping practices in years 7-9. The sample included all the lower school teachers of mathematics, science and English and a random sample of teachers from other subjects in each school. Teachers responded to a questionnaire which explored their practices in teaching classes of homogenous or mixed ability through the use of rating scales and open-ended questions. The findings showed that the curriculum was differentiated more in ability grouped classes by content, depth, the activities undertaken and the resources used. The less able were given more opportunities for rehearsal and repetition, more structured work, more practical work, less opportunities for discussion, less access to the curriculum, less homework with less detailed feedback, while work proceeded at a slower pace and was easier. Differences in pedagogy were evident in the responses of teachers who taught both mixed ability and ability grouped classes. This suggests that it is the structures themselves which lead teachers to change their teaching practices rather than their own personal teaching styles. The power of the grouping structures is further confirmed by the similarities in responses from teachers working in schools where the predominant grouping practices differed. [ABSTRACT FROM AUTHOR]