This paper investigates how local law enforcement agencies operate within a metropolitan area when there is an elastic flow of criminal activity between them. A model is developed in which a unilateral increase in local law enforcement effort has the effect of "scaring" away criminals as well as incarcerating them. Depending on the magnitude of these two effects, an MSA with many (small) agencies could wind up engaging in more or less effort - resulting in less or more crime. In a cross section of 236 US MSAs this model is tested with surprising results. Greater agency fragmentation leads to less effort, but also to less crime! This seemingly contradictory result is robust to many alternative specifications. The paper suggests that this result could happen either from some X-efficiency advantage by smaller agencies, or if criminals are not mobile and fragmentation better matches enforcement effort to local conditions. Keywords: Jurisdictional fragmentation, criminal mobility, police expenditure. JEL Classifications: H7, R5