Fluorescent nonnatural amino acids were incorporated into specific positions of streptavidin. The positions of the nonnatural amino acids were directed by a CGGG/CCCG four-base codon/anticodon pair. The nonnatural mutants with a single 2-anthrylalanine at the 22nd, 43rd, 54th, and 120th positions, respectively, were found to bind biotin. indicating that the mutants retained active conformation. The fluorescence intensities of the anthryl groups were relatively insensitive to the positions and the biotin binding when excited at 265 nm. When the anthryl group at the 120th position was excited through energy transfer from tryptophan units, the fluorescence intensity markedly decreased with biotin binding, because of a suppression of the energy transfer. Amino acids carrying 7-methoxycoumarine fluorophore were also incorporated at the 120th position. Their fluorescence quantum yields were very sensitive to the biotin binding. The high sensitivity of the coumarine-labeled streptavidin exemplifies potential applications of fluorescent nonnatural mutants for detecting specific molecules at very low concentrations.