This is the first empirical research into prevalence rates of and motivations for antigay harassment and violence by noncriminal young adults. In an anonymous survey of 484 young adults, 1 in 10 admitted physical violence or threats against presumed homosexuals, and another 24% acknowledged name-calling. Factor analyses revealed four motivational themes: peer dynamics, antigay ideology, thrill-seeking, and perceived self-defense. Compared with nonassailants, assailants held more negative attitudes toward homosexuals and reported more negative social norms among their friends. Assailants also had higher levels of masculinity ideology and social drinking. The findings suggest that many young adults believe antigay harassment and violence is socially acceptable, particularly in response to inferred sexual innuendos or gender norms violations. Because antigay behaviors are culturally normative and usually go unreported, educational outreach to adolescents and preadolescents is likely to be a more effective prevention strategy than criminal prosecutions under special hate crimes laws.