This study describes multilingual students’ authentic use of their first and second languages in a translanguaging science classroom, from a sociocultural perspective. The study is ethnographic, and has followed some lessons each month in a translanguaging science classroom at a primary school for three years. The observed lessons were documented by four video cameras and four audio recorders, while field notes and different types of students’ texts and other teaching materials were also collected. In order investigate how language operates, and to realise the meaning semantically, we analysed the student’ use of both first and second language to tie paradigmatic relations, and how they move in linguistic loops between languages and discourses. The results illustrate the ways in which a translanguaging science classroom constitutes a resource in joint negotiations of the scientific content and its related language for multilingual students, and benefits the students’ ability to relate and contextualise the science content to prior experience. The creation of translanguaging science classrooms, in which students’ experiences and diverse cultural and linguistic resources interweave with school science, and in which multilingual students are enabled and encouraged to use all available language resources, has important implications for science education. © 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group