The resonance Raman spectra for Pseudomonas aeruginosa Met121Glu azurin have been measured at wavelengths throughout the 600-nm absorption band at low and high pH. The spectra of the mutant at pH 3.5 and 7.0 are identical. Analysis of the 600-nm absorption band and resulting resonance Raman excitation profiles under both pH conditions indicates that the excited-state distortions in this mutant are smaller than those in the native protein.