Writing-to-learn activities in science classrooms can have an impact on student learning. This study sought to examine if the audience for which students write explanations of biology concepts affects their understanding of these concepts. One hundred eighteen Year 9/10 biology students from four classes participated in the study. There were four different audiences: teacher, younger students, peers, and parents. Students' writing for peers or younger students performed significantly better on conceptual questions than students writing for the teacher or the parents. (Contains 1 figure and 4 tables.)