Abstract General practitioners (GPs) play a key role in reducing the hidden HIV-epidemic, but many diagnostic opportunities are missed in primary care. This study aimed at informing the development of an HIV-testing intervention for GPs in Flanders (Belgium) using formative research with a participatory approach. Through the active involvement of an advisory board and 16 group discussions with 122 Flemish GPs, GPs’ current HIV-testing practices and perceived practical relevance of 2 distinct HIV-testing strategies (i.e. provider-initiated testing of key populations and indicator condition-based testing) were explored in terms of their relevance and feasibility in routine primary care. Self-reported HIV-testing practices revealed that most tests performed were patient-initiated, pretest counseling was rarely done, and post-test counseling was offered mainly for patients with an HIV-diagnosis. GPs reported multiple barriers to provider-initiated HIV-testing, i.e. personal discomfort, fear of offending their patient, limited knowledge of benefits of early HIV-diagnosis, misconceptions about HIV-risks, lack of guidelines and time. Difficulties to identify patient’s sexual orientation or ethical concerns were mentioned as barriers for target group-based HIV testing. GPs assessed the current list of 64 indicator conditions as too difficult to integrate in routine care, deeming a reduced list of GP-relevant conditions as more feasible. Combined strategies (i.e. target group- and indicator-based testing) supported by official screening recommendations were perceived as successful strategies for provider-initiated HIV-testing in primary care. This formative research delivered qualitative evidence for the development of an HIV-testing intervention for primary care settings.