The paper first develops a model of the screening decision as a process of categorization. The organizational context of inspectors' exercise of discretion is then outlined; and empirical data obtained through interviews with inspectors regarding screening decisions made are presented and discussed. The author proposes a model of the screening decision as essentially a process of categorization which, with experience, becomes increasingly automatic and skill-based, conducted largely unreflectively. The study focuses on the theoretical, practical, and methodological implications of the automatic, unreflective character of some routine decisions. The findings suggest that the decision strategies that make the routine exercise of discretion humanly possible may have necessary and important implications for the control of discretion and for the implementation of agency policy. The study also aims to contribute to discussion of the mechanisms whereby broader legal and organizational concerns affect the exercise of discretion in individual cases. 6 tables, 19 notes, and 40 references