Effects of charcoal rot of cotton, caused by Macrophomrha phaseofr'na, on fiber quality of 11 commercial cotton cultivars (Gossypr'um barbadense) were evaluated outdoors in a natural clay loam soil. The soil was uninfested or infested with the fungus. At the end of the growing season, lint was obtained and subjected to the following tests: Fiber length at 50% BL, fiber length at 2.5% S.L., fiber length uniformity ratio (FLUR), strength at zero gauge, strength at 1f8 gauge, fiber strength uniformity ration (FSUR), elongation, stiffness, toughness, micronaire reading, maturity ratio, hair weight, degree of yellowness, reflectance, Congo Red, pH value, and fiber sugar content. Analysis of variance showed that cultivar, treatment, and cultivar at treatment interaction were significant or very highly significant source of variation in almost all the tested properties. Cultivar (genotype) accounted for most of the explained (model) variation in physical and mechanical characters, while treatment (environment) accounted for almost all the explained variation in chemical properties. Due to the significance of cultivar x treatment interaction, an interaction least significant difference was used to compare between means of noninfested and infested soils within cultivars for each of the tested properties. These comparison showed that elongation, FLUR, and FSUR were the least sensitive properties to M. phaseoir'na infection because elongation was not adversely affected in any of the tested cultivars, while FLUR or FSUR were adversely affected in only two cultivars. On the other hand, fiber strength at zero, and chemical properties were the most sensitive properties to M. phaseoir'na infection because fiber strength at zero was adversely affected in 10 cultivars, while any of the chemical properties was adversely affected in all the tested cultivars. The other properties were adversely affected in a number of cultivars ranged from 4 to 10. Giza 76 and Giza 85 were the least susceptible cultivars to deterioration by M. phaseoir‘na infection. Thus, Giza 76 was adversely affected in 6 properties, while Giza 85 was adversely affected in 5 properties. On the contrary, Giza 45, Giza 80, Giza 83, Giza 86, and Dendera were the most susceptible cultivars to deterioration because the number of adversely affected properties in these cultivars were 12, 12, 12, 11, and 15, respectively. The number of adversely affected properties of GiZa 75, Giza 77, or Giza 84 was 8 and increased to 10 in Giza i0.