About 75 percent of U.S.-trained, noncitizen PhDs in science and engineering work in the United States after graduation, and 54 percent of those who stay take postdoctoral positions. The probability of postdoctoral participation is substantially higher for temporary visa holders than for permanent visa holders because of visa-related restrictions in the U.S. labor market. To identify the causal effects of visa status on entry into a postdoctoral position, this paper uses a unique shock to visa status generated by the Chinese Student Protection Act of 1992. Eligibility for the act is used as an instrumental variable for visa status. Two-stage least-square estimates show that permanent visa holders are 24 percent less likely to take postdoctoral positions than temporary visa holders. The effects of a permanent visa vary considerably across research fields, but for most fields, it reduces postdoctoral participation significantly. (Contains 27 footnotes, 6 tables, and 1 figure.)