. This work reports on the bioleaching of spent NiMo catalyst using Aspergillus niger. As-received and decoked catalysts of various sizes (as-received, 180-120 µm, 120-45 µm and <45 µm) were examined. Chemical analysis of the spent catalyst confirmed the presence of heavy metals including Al (38.2%), Ni (3.36%), and Mo (12.9%). SEM-EDX analysis revealed a change in the metal distribution within the spent catalyst following the decoking process. In one-step bioleaching where the fungus was inoculated together with the catalyst, fungal growth was not observed. In 2-step bioleaching where the catalyst was added three days after the start of the fungal incubation, the highest Mo (87.3%) and Al (17.20%) leaching efficiencies were obtained from the catalyst of smallest particle size (i.e. decoked grounded; <45µm) while the highest Ni leaching efficiency (94%) was achieved from the as-received spent catalyst.