Chitosan (CTO) films containing ammonium 1,1'-disulfopropyl-9-aminothiacarbocyanine (KP2) were fabricated by dropping an aqueous solution of CTO and KP2 on the quartz plate. Although KP2 molecules take a monomeric form in aqueous solutions, most dye molecules form the H-aggregate in the presence of CTO, as evidenced by an exceptionally sharp H-band. The formation of the H-aggregate was further promoted in the CTO films. In contrast, the formation of the H-aggregate reported thus far has been accompanied with the coexistence of large amounts of dye monomers and oligomers. We could devise, for the first time, a reproducible method for preferred formation of the H-aggregate in polymer films.