Crack propagation tests on a bulk metallic glass, Zr55Cu30Ni5Al10, were conducted either in aqueous sodium chloride (NaCl) solutions or deionized water. Crack growth experiments were conducted under cyclic loading at a stress ratio of 0.1 or 0.5 under a loading frequency of 20 or 1.0 Hz. The experiments were also conducted under a sustained load. Although the crack growth rate in deionized water was almost identical to that in air, the rate in NaCl solution was much higher than that in air even in a very low concentration of NaCl such as 0.01%. In 3.5% NaCl solution, the time-based crack propagation rate during cyclic loading, da/dt, was determined by the maximum stress intensity factor, Kmax, but was independent of the loading frequency and the stress ratio, and the rate was almost identical to that of environment-assisted cracking under a sustained load.