Room temperature experiments on a quartzite with 7% porosity show that with increasing pressure there is a transition from brittle faulting to ductile cataclastic flow at about 600 MPa. In contrast, experiments on nonporous quartzite show only faulting up to 1000 MPa. The transition to cataclastic flow in the porous quartzite is due to the changing influence of the pores. Microstructural observations and elastic theory suggest that the transition occurs at the pressure where the differential stress producing a compressive stress concentration sufficient to initiate pore collapse becomes less than that producing a tensile stress concentration sufficient to nucleate axial microcracks at the tops and bottoms of pores. Cataclastic flow of the porous quartzite is only transient. Deformation remains distributed and accompanied by work hardening only as long as pore collapse continues (to -16% strain); at higher strains, microcracking produces a net dilatancy, which causes strain weakening that leads to faulting (at -26% strain). At a confining pressure of 750 MPa the faults are stable, whereas at 1000 MPa they are unstable. The results indicate that porous rocks may undergo limited ductile deformation at shallow levels in the crust, but higher strains will cause deformation to localize. ¿ American Geophysical Union 1989