The naphthol-modified beta-cyclodextrins 1 and 2 have been synthesized as novel chemosensors for detecting contaminants in drinking water. They exhibit a broad fluorescence band with a peak around 505 nm for 1 and 485 nm for 2, arising from anion species of the excited naphthol unit. Upon the addition of contaminant guests, the fluorescence intensity of 2 decreased, while that of 1 increased. These fluorescence changes may be related to the guest-induced location change of the naphthol moiety from the inside to the outside of the cyclodextrin cavity. They exhibited high sensitivities for carbon tetrachloride, among several alkyl halides, as well as for odorants such as geosmin and 2-methylisoborneol. These results demonstrate that 1 and 2 will be useful for monitoring the purity of drinking water.