Time-resolved fluorescence decay measurements of individual erabutoxin b protein crystals showed evidence of conformational heterogeneity for the single tryptophan residue, Trp 29. The relative proportion of each decay component was dependent on the angular orientation of the crystal with respect to the polarization of the incident excitation laser beam. The experimental data demonstrate the existence of rotamers of the tryptophan residue side chain in the protein crystal. Model functions which simulate the orientational dependence of the decay component relative proportions were consistent with this rationalization. These results confirm the "rotamer" model for the interpretation of tryptophan fluorescence decay of proteins in solution.