This study examines how academic assertiveness in junior high school students affects conceptual change and the degree to which their assertiveness affects the conceptual change of partners paired with them for a series of activities using a computer simulation of the human cardiovascular system. The sample was a group of 90 eighth-grade students in a life science class. Students were randomly assigned to dyads, and dyad members used the simulation as an exploratory vehicle for resolving questions about 12 cases. A measure of academic assertiveness was completed for each student by the teacher. Ratings of assertiveness by the teachers were consistent with behaviors exhibited by the students. Typically academically assertive behaviors were making suggestions to partners, challenging partners' interpretations, and physically appropriating the computer's keyboard and mouse. Regression analyses showed that assertiveness did not play a role in the conceptual change posttest score, but the assertiveness of an individual's partner was inversely related to the individual's score, even after pretest score and self-efficacy beliefs were accounted for. Socially assertive behaviors may result in greater attention to the task and are also indicative of greater intellectual engagement. These results suggest that students with similar levels of assertiveness should be paired for dyad work. Observations showed that less assertive students were passive observers, and unless they were paired with students with similar levels of assertiveness, they would not be prompted to take any initiative in a relatively ill-defined learning environment where shared interpretations and joint decisions are necessary. (Contains 1 figure, 4 tables, and 41 references.) (SLD)