The primary sources of income for organized criminal gangs are gambling, bookmaking, drug trafficking, loan sharking, commissions illegally taken through interventions in civil affairs, and the levying of tributes on businesses. A sample of 347 gangsters randomly selected from among those arrested by the police from January to February 1991 participated in structured interviews regarding their perceptions of which citizen activities were most effective in obstructing a gang's criminal activities. Some of the most effective preventive activities mentioned by gang members were the draining of gang revenue sources, the obstruction of gang interventions in civil affairs, refusals by businesses to conduct transactions with gangs, and citizen activities that reduce the demand for illicit goods and services supplied by gangs. Citizen activities that remove gang offices or check the new construction or reconstruction of gang offices in a community are also effective in the obstruction of gang activities. If a gang office is removed and gangsters are forced to move out of a community, the gang loses authority in the underworld and decreases in power. Measures that prevent juveniles from joining gangs also promise to undermine the strength of gangs over time. (Author abstract modified)