Writing in science education is considered to have great potential for language and science learning. However, the ways in which teachers and their students interact as writing communities in science lessons has so far rarely been studied in depth, particularly regarding the norms which guide cooperative writing. The present article remedies this by presenting the results of a study on the writing norms observable in cooperative writing tasks in science lessons in Swiss primary schools (3rd, 4th and 5th grade). A video corpus of appr. 50 hours was analysed using a combination of approaches, namely Qualitative Content Analysis and Conversation Analysis. First, the concept of norms underlying the present study is discussed and then, the methodological procedure, the data and results are presented. The writing norms reconstructed on the basis of the corpus represent six areas: 1. the way writing tasks are dealt with in science lessons, 2. content, 3. wording, 4. accuracy (orthography, morphology, syntax), 5. visual appearance (handwriting, layout) and 6. the procedure of cooperative writing itself. The article provides an overview of the reconstructed norms as well as a more detailed discussion of selected norms illustrated by corpus examples.