This study of 1,338 Spanish-speaking 1st graders examined contextual effects of bilingual programs on reading comprehension and the effect of language of instruction within these contexts. The study included 128 classrooms in 32 schools located in border Texas and in urban Texas and California. These classrooms used either English immersion or Spanish maintenance bilingual programs. Detailed observations of teachers' instructional language were made, sampled within the year. The analyses allowed classroom-level differences to be separated from student-level differences, and for Spanish and English passage comprehension to be considered simultaneously. While mean differences between programs were reduced for English passage comprehension, maintenance programs still outperformed immersion programs in Spanish. Results also indicated large program and locale covariance differences at the classroom level, implying important differences in how these programs operate in these locales. (Contains 6 tables, 3 figures, and 1 footnote.)