Language is critical in the mediation of scientific reasoning, higher-order thinking and the development of scientific literacy. This study investigated how an exemplary primary science teacher scaffolds and supports students' reasoning during a Year 4 materials unit. Lessons captured on video, teacher and student interviews and micro-ethnographic analysis of classroom discourse reveal a sophisticated repertoire of teacher practices focusing on the development of language, conceptual understanding and reasoning within a classroom culture that supports reasoned arguments. The integration of conversation threads supporting language and conceptual development, syntactical and metacognitive scaffolds, prompts and self-regulation strategies extend the opportunities for scientific reasoning. Recommendations are made for adopting strategies that enhance opportunities for reasoning in primary science lessons.