Within the framework of sociocultural theory, learning is conceptualized as participation rather than acquisition (Donato, 2000). Given the governing metaphor of changing participation as learning (Young & Miller, 2004), an important contribution conversation analysis can make to the study of second language acquisition is to detail the instructional practices that either create or inhibit the opportunities for participation (Lerner, 1995), and by extension, the opportunities for learning. This study focuses on one such practice in English as a second language classrooms—the use of explicit positive assessment—and its relevance to learning opportunities. I argue that within certain contexts these assessments tend to suppress the opportunities for voicing understanding problems or exploring alternative correct answers, both of which are the stuff that learning is made of. The analysis suggests that what is sequentially and affectively preferred may be pedagogically and developmentally dispreferred.