This research provides a detailed analysis of the inner workings of a bilingual program implemented in San Diego, California. The children in the study divide their school day between two separate classrooms, one offering instruction in Spanish and the other in English. The same students were videotaped as they participated in reading lessons in both languages. The goal of the analysis was to specify the variations in the communicative activities that constitute the reading lessons, and to pinpoint lesson features that helped or hindered the development of second language skills. Results show that teachers often structure lessons in ways that interfere with their curricular objectives. The analysis of lessons across language and ability groupings indicates that: (1) the manifestation and complexity of reading behaviors in the two classrooms are determined by the types of interactional activities, called "participation structures," in which students and teacher engage; and (2) these participation structures are influenced by organizational constraints and by presuppositions about the children's competence. It was found that the extent to which reading behaviors in Spanish transfer to or can be manifested in English depends on similarities in the organization of the participation structures found in the two classrooms. (Author/AMH)