Creep of Nextel™610 fibers was investigated at 1100°C and 100–500 MPa in air and in steam. The effect of loading rate on fiber tensile strength was also explored. The presence of steam accelerated creep and reduced fiber lifetimes. Loading rate had a considerable effect on tensile strength in steam, but not in air. A linear elastic crack growth model was used to predict the creep lifetimes from the constant loading rate data. The dependence of tensile strength on loading rate and the predictability of creep lifetimes suggest that the failure mechanism in steam was environmentally assisted subcritical crack growth. The creep-rupture data were analyzed in terms of a Monkman-Grant (MG) relationship. Monkman-Grant parameters for creep-rupture data were the same in steam and air, and predicted creep-rupture at 1100°C in both environments. A grain-size increase of about 25% was observed by TEM after 100 h at 1100°C in steam, which was about two times that observed in air.