The visible absorption spectra of the cross-section of natural fibres (cotton, jute, wool and silk) dyed with Acridine Orange hydrochloride or free base were measured by means of a microspectrophotometer. As in the case of synthetic-polymer fibres, the spectra showed monomer, aggregate and crystal band. Many void places or channels were observed in these natural fibres by scanning-electron microscopy. In general, the fibres which have weak affinity for Acridine Orange free base show crystal band. This may be because the dye affinity for fibre retards the formation of crystals.