This study was designed to investigate the effects of group presence and arousal in deindividuation, a process that is theoretically related to performance of anti-social behavior. On Halloween night, 23 selected homes in Seattle, Washington, were prepared for experimental manipulation. All homes were set up so that observers could watch children take candy from a bowl without being seen by the trick-or-treaters. In one group of homes, physical arousal was induced in the children through a game of "Simon Says." Another group induced arousal with taped ghostlike noises, and control homes used only group versus alone manipulations. Subjects were 841 trick or treaters, who were instructed to take only one candy apiece. The number of candies taken (over one) was used as the dependent measure of anti-social behavior. Results indicated that children in the group condition were significantly more anti-social than those who were alone. Both arousal conditions produced a stealing effect, but exercise produced significantly more anti-social behavior. Discussion focuses on implications for deindividuation research, based on the findings that certain factors acting together are powerful disinhibiting forces. (DP)