The emission decay of intrinsic fluorescence of the extremely thermophilic beta-glycosidase from Sulfolobus solfataricus has been investigated as functions of temperature and of iodide-quencher concentration by frequency-domain fluorometry. This protein contains 68 tryptophans and provides a matrix for correlation of the average spectroscopic behaviour with solvent exposure and local dynamics. At each temperature, the emission is very heterogeneous and interpretable in terms of quasicontinuous bimodal distribution of fluorescence lifetimes. We associate the component of the bimodal distribution to two distinct classes of tryptophanyl residues that differ in microenvironmental characteristics. Temperature and quenching experiments show that the long-lived component includes tryptophanyl residues located in buried regions with high rigidity; the short distributional component corresponds to tryptophans embedded in more flexible and exposed regions. This proposal has been confirmed by examination of the crystallographic structure. The data suggest that, at least for this protein, there is a good correlation between residue exposure and lifetime distributional components. The conformational dynamics of the two classes of tryptophanyl residues is affected differently by temperature, suggesting that the protein regions in which they are located give different contributions to enzyme properties, such as flexibility, stability and function.