The purpose of this study was to examine distinct latent classes of middle grades mathematics teachers with respect to reasoning about fractions. Survey response data came from a nationwide sample of 990 in-service middle grades mathematics teachers. The survey focused on four components of reasoning about fractions in terms of quantities: referent unit, partitioning and iterating, appropriateness, and reversibility. The mixture Rasch model analysis detected three latent classes, each with strengths and weaknesses. Chi-square tests indicated significant relationships between latent class membership and various teacher characteristics such as gender, mathematics credential, grade-level experience, and highest grade-level certification. The results extend recent advances in measuring mathematical knowledge of teachers. [For the complete proceedings, see ED606531.]