The interaction of carboxymethyl cellulose(CMC) with C.I. Basic Red 18(Red 18) in homogeneous systems was studied with absorption and circular dichroism spectra measurements. The peak of the absorption spectra of Red 18 shifted to sllorter wavelength and its intensity was reduced with an increase in poBymer/dye ratlo. The cooperative interaction of Red 18 to CMC was interpreted quantitatively in terms of the stacking aggregate theory. Stacking coefficient increased with further decrease in polymer/dye ratio, corresponding to the formation of higher cooperative interaction to CMC. Multiple induced Cotton effects in the visible spectral region were observed depending on the conditions of binding in aqueous solution. Red 18 bound to CMC had a high stacking tendency and a dye molecule partly occupied an etherificated site adjacent to those sites already occupied by other dye molecules on the helical part.