This article investigates the practice of heterogeneous grouping for reading and literacy instruction through a detailed interactional analysis focused on the least proficient readers in two mixed-ability group contexts. The resulting analysis suggests that struggling readers in heterogeneous groups may encounter the same problems often associated with their placement in homogeneous ability groups. The mixed-ability groups examined in this study were characterized by pervasive interactional differentiation, which for some students may lead to the kind of stigmatizing effects thought to result from long-term participation in low-ability groups. In addition, the low-ability students in these heterogeneous groups read less and were interrupted more often than the other students--two differences in the treatment of low-ability students that have also been associated with homogeneous ability grouping. The study thus points to the difficulty that teachers face in promoting simultaneous reading development among multiple proficiency levels. It also suggests the need for renewed research efforts focused on current instructional alternatives to traditional ability groups, as well as the experience of struggling readers within them.