This paper investigates regional variation in migration policy implementation, focusing specifically on the underexplored role of policy ambiguity. It chooses a salient case study of internal migration control implementation: the application of labour market access policies for migrants with precarious legal status in German municipal immigration offices. Studying the implementation approaches of eleven offices within one Land by means of semi-structured interviews with senior officials, the research design allows for drawing inter-agency and inter-policy comparisons. The data provides empirical evidence for the claim that the more conflictive and hence ambiguous a policy, the more importance can be placed on local determinants of implementation. Different logics (economic welfare and regulatory control logic) legitimizing more restrictive or expansive implementation are identified and linked to the broader migration policy context. Moreover, the difficult task of officials to determine applicants’ identity clarification efforts – a condition for receiving a work permit – serves as basis for conceptually distinguishing between collective and individual discretion of street-level bureaucrats.