The present study aimed to identify the role of both student- and school-level characteristics in primary school students' achievement in the science curriculum. As societies become more culturally and linguistically diverse, many students enter the classroom with a home language that is different from the language of instruction used at school. This study takes into account both the home language and literacy in the language of instruction in relation to student achievement in science subjects. Questionnaires, reading performance tests, and science achievement tests were administered to 1,761 fourth-grade students from 67 schools across Flanders (Belgium). Multilevel hierarchical regression analyses show that the home language and literacy in the language of instruction play an important role in science achievement at the student level, next to gender and socioeconomic status. Students with a home language that is different from the language of instruction experience difficulties with science subjects. Moreover, the higher students' performance on reading comprehension and self-assessed proficiency in the language of instruction, the higher their score on science achievement tests. At the school level, a school's teachability expectations are one of the key factors related to students' science achievement. Limitations of this study and future directions for research are discussed.