Background and Objectives: CATALASEs are a good scavenger of H O which degrades hydrogen peroxide into water and 2 2 oxygen. They are considered as a virulence factor that are present in both spores and hypha of fungi. There is limited data regarding CATALASE ACTIVITY in Aspergillus species. This study aimed to assess the mycelial CATALASE ACTIVITY of clinical and environmental isolates of Aspergillus niger, A. tubingensis, A. flavus, A. luchuensis, A. piperis and A. terreus. Materials and Methods: Briefly, clinical and environmental Aspergillus species were used in the current study. CATALASE ac-tivity was assessed for both groups of isolates including 13 A. flavus (12 clinical, 1 environmental), 13 A. terreus (8 clinical, 5 environmental), 26 A. tubingensis (13 clinical, 13 environmental), and 44 A. niger (25 environmental, 19 clinical) species. Fungal balls of mycelia were separated from the liquid culture and were crushed using homogenizer. The supernatants were collected and used for a CATALASE ACTIVITY assay. Results: Totally, in our study 98 Aspergillus including 45 environmental and 53 clinical isolates were assessed for CATALASE ACTIVITY. High CATALASE ACTIVITY was detected among environmental Aspergillus species (Mean= 1. 62 mU/ml) and the mean of mycelial CATALASE ACTIVITY among clinical A. terreus isolates was higher than environmental strains. Conclusion: In summary, mycelial CATALASE ACTIVITY varied among species and environmental isolates demonstrated higher CATALASE ACTIVITY. Totally a significant difference was found between clinical and environmental Aspergillus isolates.