Learning science encompasses learning the language of science, which can pose considerable challenges to students due to its specialized features and structures. Addressing the three-language problem (the need to transition between daily, general school and disciplinary languages) and the specific language demands of science thus constitutes part of the larger goal of promoting disciplinary literacy. Most studies have focused on the conceptual demands of science learning and have not examined what knowledge teachers require to support students' learning of the language of science. In this study, we focus on science teachers' knowledge of students (KS) for addressing the language demands of science. This qualitative case study has two aims: (1) to unpack a teacher's KS related to their language use in science and (2) to uncover how this KS informs teaching practices. Transcripts of teacher interviews, lesson planning sessions and lessons drawn from two Grade 4 science classrooms taught by a primary science teacher constituted the data for this study. We identify five aspects of KS (i.e. prior knowledge of and about language, difficulties with language, abilities across modes of language and across subject areas and learning progress) that inform four distinct teaching practices. This case study highlights and unpacks a dimension of KS that is often overlooked in the current literature on pedagogical content knowledge (PCK) for science teaching. The implications to teachers' PCK, science teaching and teacher professional development are discussed in light of the findings.