We present evidence that efforts to make law enforcement activity more salient at minimal cost greatly reduce illegal behavior. We conduct a natural field experiment into illegal disposal of household garbage on the sidewalk. In the control condition, law enforcement officers patrol the streets, and occasionally look for name and address identifiers in illegally disposed garbage bags. In a number of randomly selected locations, in addition to the regular surveillance activities, illegally disposed garbage bags were marked with bright yellow warning labels saying ‘This bag has been found by law enforcement. 90 euro fine for illegal disposal’. We find residents to respond to the warning labels by reducing illegal disposal of garbage. We also find residents to become less likely to leave name and address identifiers in disposed garbage bags, which subverts law enforcement.