Activity structures such as classroom lessons and initiation-reply-evaluation sequences are important cultural tools that help students and teachers accomplish everyday activity, but they are not well adapted to open-ended inquiry conducted by students in small groups with teacher guidance. In this research, I identified alternative activity structures that better enable teachers to scaffold children's performance of open-ended projects involving artifact construction. A case study and discourse analysis from an earth science class show how dialogic activity structures at 2 time scales support students in learning the discipline of science. At the scale of a multiweek unit, an activity structure embodies aspects of science practice such as assembling data to support claims. At the scale of verbal exchanges, action negotiation dialogues, student questioning dialogues, and action feedback dialogues help the teacher to guide students in their work while requiring that students maintain agency. I discuss implications for the design of project-based learning environments.