The deformation of 9.4 mol% magnesia-partially-stabilized zirconia under compressive loads up to 1225 MPa was studied using mechanical testing with in situ neutron diffraction. The material shows obvious plastic deformation at applied stresses in excess of an estimated critical stress of 925 +/- 20 MPa. Most of the accumulated strain occurred by transient room-temperature creep. Plastic deformation was associated with considerable stress-induced tetragonal-to-monoclinic transformation. The volume change calculated from the strain gauges correlates well with the amount of t --> m transformation observed. Unlike previous studies of Ce-TZP and Y-TZP, ferroelasticity was not observed, nor was the t --> o transformation observed. Minor microstructural changes were noted, including an increase in the root mean square internal strain of 0.05%, commensurate with an increase in internal stress of similar to100 MPa. It would appear that transformation selectivity was exercised with the transformation occurring first in tetragonal crystallites favorably oriented to the applied stress. The stress-induced monoclinic phase therefore exhibits a strong preferred orientation. Comparison is made with the other commercially interesting zirconia ceramics, Ce-TZP and Y-TZP, which have been studied using the same techniques.