The creep behaviour at 1370 degrees C (2500 degrees F) of yttria-doped, hot isostatically pressed silicon nitride was examined as a function of residual alpha phase content. The pre-test silicon nitride materials had either 30% or 40% alpha phase content. The creep resistance was found to increase as the residual alpha phase content decreased. For equivalent times and stresses, the higher alpha-containing silicon nitride accumulated more creep strain and exhibited faster creep rates. The residual alpha phase decreased as a function of time at 1370 degrees C and converted to beta phase; it was also found that the alpha to beta phase transformation rate was enhanced by stress. In the absence of stress, the kinetics of the alpha to beta phase transformation at 1370 degrees C followed a first-order reaction. If a first-order reaction was assumed for the alpha to beta phase transformation in the presence of stress at 1370 degrees C, then the magnitude of the reaction rate constant for this transformation was twice as large for tensile stresses equal to or greater than 130 MPa than for the reaction rate constant describing the transformation with no applied stress.