The present investigation was carried out at Sakha Agricultural Research Station over three cotton growing seasons 1997, 1998 and 1999. The two Egyptian cotton varieties, "Giza 45" and "Giza 76" belonging to Gossypium barbadense L. as well as their spontaneous induced offtypes, were used in this investigation. The results showed significant differences between the original parents and their derived offtypes for fibre characters in both varieties indicated that these changes appeared to be genetic alterations, one or more, mutant locus. Both naked-creamy parental offtypes showed higher values of fibre fineness and degree of yellowness, but lower lint reflectance value than the original varieties. The F₂ frequency distribution curves of the two crosses were characterized by a kind of unimodality confirmed the polygenic nature of genes controlling such changes in lint quality. The additive gene effects were the prevailing in governing the lint quality changes in both crosses besides, some various types of non allelic interaction only in the cross, Giza 45 x naked-creamy offtypes, reflecting that the induced naked-creamy offtypes possessed different genetic system varied in their action. The detected high heritability values in both crosses for the studied fibre quality characters indicated the possibility of selection for better lint quality values and eliminate such offtypes in early segregation generations.