[Structure: see text] Carboxyfluoresceins are widely utilized as fluorescence labeling reagents, but we recently found that their emission intensity is markedly decreased after esterification. On the basis of our hypothesis that the fluorescence decrease is due to a donor-excited photoinduced electron transfer (d-PeT) process, we have developed novel carboxyfluorescein derivatives in which the d-PeT process is hampered, and the emission intensity is not decreased upon esterification. These novel dye derivatives display high quantum yields and are expected to be useful as labeling agents.