Computer environments could support students in engaging in cognitive activities that are essential to scientific practice and to the understanding of the nature of scientific knowledge, but that are difficult to manage in science classrooms. The authors describe a design for a computer-based environment to assist students in conducting dialectical activities of constructing, comparing, and evaluating arguments for competing scientific theories. Their choice of activities and their design respond to educators' and theorists' criticisms of current science curricula. They give detailed specifications of portions of the environment. © 1994 John Wiley & Sons, Inc.