This study examined students' active roles in constructing knowledge when learning new information in the domain of biological evolution. A computer-based connectionist methodology was developed to provide a way to present students with new information while the experimenter provides probe statements congruent with or contradictory to the student's beliefs. Fifty-four students in grade 9 and 54 students in grade 12 each participated in 1 of the following 4 conditions: (1) individual-assimilation; (2) individual-conflict; (3) peer-assimilation; and (4) peer-conflict. Pretests and posttests and protocol analysis identified knowledge changes and knowledge processing activity. Path analysis suggested that only strategy exerts a strong direct effect on conceptual change and mediates the effects of age, prior knowledge, and conflict. Protocol analysis showed that students using a direct assimilation approach could use different strategies to assimilate contradictory information even though it represented something quite different from what they believed. The peer interaction condition did not produce significant effects in fostering conceptual change, although there were indications that group effects on conceptual change were greater for older students in the conflict condition. A table showing hypothetical inputs representing students' beliefs in each of the four conceptions and the corresponding weight activations generated by the network is appended, and five figures illustrate student processing. A 26-item list of references is included. (SLD)