We examined how the process of cultural formulation contributes to diagnostic assessment of patients with psychotic disorders at a specialized Cultural Consultation Service (CCS). Specifically, we investigated the reasoning process used to resolve uncertainty of psychotic disorder diagnosis in African immigrant patients referred to the CCS for assessment of possible psychotic disorder. Qualitative thematic analysis of 23 clinical case conference transcripts was used to identify clinicians’ reasoning styles. Use of the CF appears to facilitate the emergence of a rule-governed reasoning process that involved three steps: (i) problematize the diagnosis of the intake ‘psychosis’ symptoms or behavior; (ii) elaborate explanations as to why the symptoms or behavior may or may not be psychosis; and (iii) confirm the diagnosis of psychosis or re-interpret as non-psychosis. Prototypes and exemplars drawn from previous experience in intercultural work featured prominently in clinicians’ reasoning. Prototypes were crucial in diagnostic decision-making and appear to be important sources of both clinician expertise and bias, and may need to be targeted specifically in cultural competence training.