Two-stage partial nitritation/anammox (PN/AMX) processes have been pointed out as a feasible configuration for achieving mainstream anammox. For two-stage configurations, stable partial nitritation has been reported feasible in granular sludge reactors. This study aimed to explore the operating conditions involved in the development of an autotrophic aerobic granular sludge using floccular sludge as inoculum. The influence of different parameters such as free ammonia concentration, settling time, superficial gas flow velocity and ammonium oxidation rate was investigated. Enhancing ammonium oxidation activity since the early phase of the operation (i.e. using conventional activated sludge as inoculum enriched with a fraction of a floccular nitrifying biomass) promoted a fast development (ca. 30 days) of an autotrophic aerobic granular sludge performing stable nitritation. When the seeded sludge presented a low nitrifying activity (i.e. not enriched), the increase of the air-flow rate triggered the formation of an autotrophic aerobic granular sludge since ammonium oxidation activity was promoted. Contrarily, imposing low settling times or strong free ammonia inhibitory conditions were shown to negatively influence the achievement of high ammonium oxidation rates, hampering the development of an autotrophic aerobic granular sludge. This study demonstrated the importance of ensuring high ammonium oxidation rates for the proper development of an autotrophic partial nitritation granular sludge.