The fungi are an exceptionally useful model in elucidating the cell functions under extreme conditions (temperature, salinity, and pH values) and mechanisms underlying adaptation to those extreme environmental conditions. Here, Chaetomium globosum was examined for its adaptation mechanism under alkaline stress using antioxidant properties. The aqueous extracts of C. globosum exhibited different levels of antioxidant activity in all the in vitro tests such as alpha,alpha-diphenyl beta-picrylhydrazyl (DPPIT center dot), hydrogen peroxide, hydroxyl radical scavenging assay (HRSA), ferric reducing/antioxidant power (FRAP) assay, metal chelating assay and (beta-carotene-linolic acid model system. The antioxidant capacity of marine fungi showed an increase in activity with increase in stress. In addition, the production of intra and extracellular antioxidant enzymes of the fungus at various pH stresses were analyzed and discussed for their possible role in the stress mechanism. The present study elucidates that the scavenging activity is one of the protective mechanisms developed to avoid the deleterious effect of stress. Furthermore, the in vitro assays also clearly indicate that fungal extracts are significant source of natural antioxidant, which might be helpful in preventing the progress of various oxidative;tresses. (c) 2010 Elsevier Ltd. All rights reserved.