This study uses a new source of data to assess the degree to which the Immigration Reform and Control Act (IRCA) deterred undocumented migration from Mexico to the United States. Data were collected from migrants interviewed in seven Mexican communities during the winters of 1987 through 1989, as well as from out-migrants from those communities who subsequently located in the United States. We conduct time-series experiments that examine changes in migrants' behavior before and after passage of the IRCA in 1986. We estimate trends in the probability of taking a first illegal trip, the probability of repeat migration, the probability of apprehension by the Border Patrol, the probability of using a border smuggler, and the costs of illegal border crossing. In none of these analyses could we detect any evidence that IRCA has significantly deterred undocumented migration from Mexico.