Deterioration in fibers of the cotton cultivars Giza 80, Giza 85, Giza 86, Giza 89 and Giza 90 (long staple), and Giza 88 (extra long staple) caused by Alternaria sp., Fusarium semitectum, Trichothecium sp. Penicillium sp., Trichoderma sp. Rhizopus, Aspergillus flavus, Cladosporium herbarum, F. monoliforme, and Nigrospora sp. Was evaluated under pure culture conditions. The tested properties were upper half mean, uniformity index, short fiber index, Brightness, Yellowness, trash no., maturity, micronaire value, fiber strength, elongation, cellulose, reducing sugar content, and fiber damage index. Cultivars, fungi, and cultivars × fungi interaction were all very highly significant sources of variation in all the tested properties. Cultivars were the most important source of variation in most of the tested properties. Due to the significance of the cultivars × fungi interaction, a least significant difference (LSD) was used to compare the individual fungal means within cultivars for each of the tested properties. These comparisons showed that most of the tested properties tended to decline as a result of fungal infection; however, the magnitude of decline varied from one cultivar to another. The present study clearly demonstrated that cotton cultivars were much more important than fungal isolates in determining the level of deterioration in most of the tested properties. This result implies that the deleterious effects of the cellulose-degrading fungi on quality of fibers could be considerably reduced if resistant of cotton cultivars to these fungi is effectively enhanced.