The fluorescence of 2-(5-amino-2-methoxyphenyl) benzothiazole (1) is affected by the solvent polarity and by the protonation; red shifts of the fluorescence are observed with an increase of the solvent polarity, and the fluorescent quenching is caused by the protonation at the amino group.  Because the fluorescence intensity is sensitive to the association at the amino group, 1 is expected to work as a framework for fluorophores responding to metal cations by the association at the amino moiety. The benzothiazole 2, having the pyridylmethyl-modified amino group as a powerful association site, is found to respond to Mg²⁺, giving rise to blue shift of the fluorescence, and to Zn²⁺, Hg²⁺, Ni²⁺, Cu²⁺, Ag⁺, and　H⁺, causing fluorescent quenching.