We designed and fabricated a true triaxial loading system and used it to determine deformational and strength characteristics of the amphibolite penetrated by the superdeep hole drilled in the Bohemian massif of southeastern Germany under the German Continental Deep Drilling Program (KTB). Amphibolite is found between 3200 and 7300 m and thus the dominant rock in this 9100-m boring. Our loading system enables the application of three unequal principal stresses to a rectangular prismatic rock specimen. During a test we maintained the least principal (&sgr;3) and the intermediate (&sgr;2) stresses constant and increased the major principal stress (&sgr;1) until brittle failure occurred, in the form of a fracture steeply dipping in the &sgr;3 direction. Typically, for the same &sgr;3 level the amphibolite compressive strength increased substantially with the magnitude of &sgr;2, demonstrating the inadequacy of Mohr-like failure criteria that ignore the effect of the intermediate principal stress on rock strength. We found that a general criterion for the amphibolite could be expressed in the form of a power function relating the octahedral shear stress at failure to the mean normal stress acting on the plane containing the fracture. With respect to deformation, we established that for the same &sgr;3 the onset of dilatancy increases significantly with the magnitude of &sgr;2. Thus the intermediate principal stress appears to extend the elastic range of the stress-strain behavior for a given &sgr;3 and hence to retard the onset of the failure process. Scanning electron microscopy observations of the failure process reveal that microcracks develop mainly parallel to &sgr;2 direction, as the intermediate stress grows beyond &sgr;3, localizing in close proximity of the eventual main fracture. ¿ 2000 American Geophysical Union