The purpose of this study was to investigate the effects of conceptual assignments and conceptual change discussions on students' achievement and misconceptions about force and motion. The study was conducted with 6 physics teachers and their 18 classes, consisting of 396 high school physics students. The teachers administered the Force Misconception and Force Achievement Tests to their physics classes as a pretest. The results obtained were used to match the 18 classes statistically. Students assigned to the conceptual assignment protocol completed five conceptual assignments about force and motion. Students assigned to the discussion method participated in conceptual change discussions. At the end of the 8-week treatment period, the same tests were administered to all students as a posttest. The data were analyzed by using multivariate analysis of covariance, followed by protected univariate F test and stepdown analysis. The statistical results showed that the conceptual change discussion was an effective means of reducing the number of misconceptions students held about force and motion: The conceptual change discussion was also found significantly effective in improving students' achievement in force and motion. (C) 2002 Wiley Periodicals, Inc.