Inquiry-based science instruction has been proposed as an optimal learning environment for language-integrated teaching. While its potential for developing both content knowledge and language skills has been shown for students with limited language proficiency, research focusing on mainstream classrooms has mainly considered domain-specific learning. Despite the effectiveness of inquiry-based science instruction for student outcomes, research on the role of specific aspects of instructional quality is limited. Addressing this research gap, the present study investigates the relationship between teachers' instructional support during inquiry-based science classes and elementary school students' learning gains in science content knowledge and academic language proficiency. Multilevel regression analyses are based on data from 459 German elementary school students from Grades 3 and 4 who participated in a longitudinal intervention study that took place over one school year. Our findings indicate a strong impact of students' prior knowledge on learning outcomes and compositional effects for the language-related measures. Relations between teachers' instructional support, as measured by the Classroom Assessment Scoring System (CLASS; Pianta et al., 2008), and students' learning outcomes did not emerge. In interpreting the results, the overall mediocre quality of teachers' instructional support needs to be considered.