The stability of the acidic compact state of apomyoglobin toward the denaturant action of guanidinium hydrochloride and temperature was studied by examining the effects induced on the intrinsic tryptophanyl fluorescence and that of the adduct formed with 1,8-anilinonaphthalenesulfonate (ANS). The results indicated that the disorganization of tryptophanyl environments is caused by a cooperative discrete molecular transition, thus contrasting the assumption that the acidic compact form of apomyoglobin might be a molten globule state. The unfolding of the ANS binding regions was found to involve, at least, two stages over a wide range of denaturant concentrations.