Fracture toughness studies are conducted on a Zr-based bulk metallic glass at room and low (77 K) temperatures. Samples, cast by different processing techniques, contain either crystalline defects or not. For SENB specimens containing 30 μm root radius notches, results indicate that a perfect glass without quenched-in defects is ductile at room temperature and brittle at low temperature (toughness of 72 MPa.m0.5). On the contrary, imperfect glasses are brittle at room temperatures (toughness of 50 MPa.m0.5) and brittler at low temperature (27 MPa.m0.5). Fractographic observations clearly show the embrittlement caused by the dendrites at these two temperatures. These results are discussed on the basis of previous fracture studies with respect to the oxygen content in the samples.