The effects of Aspergillus niger H-18(AN) and A.fumigatus E-29(AF) on the deterioration of test materials were examined. 1. When the fungi were inoculated to the cotton fibers placed on Czapeck agar, malt extract agar, and potato agar, they grew best on potato agar which in that context was chosen as a basal medium for deterioration studies. The tensile strength of ontton fiber on which AN was grown for 30 days decreased by 33.9% with a concommitant increase in elongation by 43.1%. 2. Cotton fabric lost the weight by 1.35%, when inoculated with AN while it lost the weight by 0.86% when inoculated with AF. 3. When AN and AF were inoculated separately on cotton fibers, tensile strength decreased by 52.8% and 43.3%, respectively, with concommitant increases in elongation by 34.2% and 29.4%, respectively. 4. The cotton was damaged more severely by AN than AF, when they were observed by scanning electron microscope.