Mo-rich Mo-Si-B multiphase alloys are currently being explored for their potential as high-temperature structural materials for components in hot sections in aircraft engines. In this paper, we present crack growth behavior in one such two-phase alloy consisting of a Mo solid solution matrix in which is dispersed approximately 40 volume percent of the Mo5SiB2 (T2) phase. Crack growth under monotonic and cyclic loading is considered over a temperature range spanning 20°C to 1400°C. The effects of loading rate (in monotonic loading) and dwell times at maximum stress (in cyclic loading) at high temperatures on crack growth were examined to understand the contribution from creep. Results confirm a gradual increase in fracture toughness upto 1000°C, beyond which the increase is more substantial with temperature; fatigue susceptibility was also observed in excess of 900°C and crack-tip-stresses-driven microstructural instability is evident at 1400°C. At this temperature, slow loading rates or dwell times at maximum stress lead to crack-tip recrystallization and creep cavitation that together degrade the material's properties.