An agency's culture shapes its lawmaking. Under certain conditions, agency culture dominates decision-making so strongly that it mutes the influence of those factors that administrative law scholars have traditionally focused on including presidential will, judicial oversight, internal resistance, and public opinion. We call this undertheorized phenomenon "zealous administration." The immigration enforcement bureaucracy has vast discretion to remove unauthorized immigrants from the United States. Current immigration policies-such as indiscriminate deportation, family separation, and harsh detention-represent the most prominent example of zealous administration in the federal government. This Article focuses on that bureaucracy to plumb the causes and effects of zealous administration and to explore ways to limit it. Zealous administration manifests in three principal ways. First, the agency engages in hyper-regulation-the exercise of authority in indiscriminate, pervasive, and performative modes. Second, the agency is politically resilient-it is uniquely impervious to influence from the President, pressure from other government entities, public disapproval, and internal dissent. Third, zealous administration-once it has taken root in an agency and absent some powerful intervention-will grow over time and coopt for its mission other agencies sharing the same regulatory space. The immigration enforcement bureaucracy's zealous administration complements President Trump's aggressive agenda, but it is not merely a product of it. Zealous administration in that bureaucracy has deep structural roots long predating the current administration. Neither the reigning presidential-control view of administrative lawmaking, nor the alternative deliberative-democratic view can fully account for it. This Article fills the gap by drawing on classic public choice theory to construct a model of immigration enforcement as regulation. It concludes that taming zealous administration requires policymakers to focus on redirecting bureaucratic incentives, redesigning institutions, and expanding judicial review.