The Pyrococcus furiosus endo--1,3-glucanase belongs to the subfamily of laminarinase, which can be classified as "all proteins" as confirmed by deconvolution of far-UV CD and FTIR spectra. The persistence of a significant amount of tertiary structure in 7.9 M GdmCl, as indicated by near-UV CD spectroscopy, accompanied by a red-shift of the maximum fluorescence emission wavelength is a peculiar property of this hyperthermophilic endoglucanase. The possibility to observe tertiary structure elements under extremely denaturing conditions is notable and is limited to only a few examples. The unusual resistance toward guanidinium chloride denaturation is paralleled by a notable stability at extremely low pH and at high temperature. The analysis of the protein spectral properties indicates that the secondary structure elements are preserved down to pH 1.0 and up to 90 C at pH 7.4 and pH 3.0. The study of the conditions that determine the persistence of residual structure at high denaturant concentration and the examination of these structures are particularly interesting because these state(s) may be preliminary or coincident with the coalescence of protein aggregates or to the formation of amyloid-like fibrils, and they may serve as seeds of protein folding.