THIS ARTICLE DISCUSSES THE CONDITIONS WHICH AFFECT HOW A GROUP OF BYSTANDERS RESPOND TO A CRIMINAL ACT AND DEVELOPS A MODEL CONCERNED WITH THE DETERMINANTS OF SPONTANEOUS VIGILANTISM. IT IS SUGGESTED THAT, AS A RESPONSE TO AN ANTISOCIAL ACT, THEREFORE, SPONTANEOUS VIGILANTISM MAY BE EXPECTED WHEN FIVE CONDITIONS EXIST: (1) AFTER A CRIME HAS BEEN COMMITTED OR A RUMOR EXISTS INDICATING THAT A CRIME HAS BEEN COMMITTED, WHERE THE INTERPRETATION OF THE INCIDENT IS UNAMBIGUOUS AND CLEAR, AND WHERE THE OPINION OF THE BYSTANDERS OR COMMUNITY IS UNANIMOUS CONCERNING BLAME FOR THE CRIME; (2) IN HIGHCRIME AREAS WHERE CRIME IS OF SPECIAL CONCERN TO THE RESIDENTS; (3) IN ETHNIC AREAS WHERE PEOPLE DEVELOP A SENSE OF EMPATHY AND COMMUNITY; (4) IN AREAS WHERE THE POPULATION FEELS THAT THE LEGAL PROCESS IS INADEQUATE IN DEALING WITH THE CRIME PROBLEM; AND (5) IN POOR AREAS WHERE CONDITIONS OF GOAL BLOCKING OR FRUSTRATION EXIST. (AUTHOR ABSTRACT MODIFIED)