Research on elementary schoolteachers' beliefs about dual language development and its relation to English learners' (ELs) achievement is scant, despite the increasing representation of ELs in US classrooms. To address this gap, this study investigates: (1) teacher beliefs about ELs' dual language development and (2) associations between teacher beliefs about ELs' dual language development and ELs' English reading comprehension achievement. Principal component analyses revealed six teacher belief factors about dual language learning, and multilevel analyses suggested that teachers' asset view and bilingual development understanding of dual language learning and development positively predicted English reading comprehension, whereas lenient expectations and English-only achievement showed an opposite relation.