The Mode I fracture toughness of fibrous SiC and mullite materials with 60%-70% porosity was determined under ambient conditions using the double-torsion test method. The fracture toughness values obtained were 0.27 and 0.34 MPa center dot m1/2 for fibrous SiC and mullite, respectively. These fibrous ceramics exhibit greater fracture resistance than porous monolithic ceramics with similar levels of porosity. A higher degree of cross-linking between the fibers led to further improvement in the fracture characteristics of the fibrous material. The differences in fracture behavior can be explained by the microstructural differences between these two classes of materials. The combination of high porosity and high fracture toughness for fibrous materials makes them ideal candidates for applications in diesel particulate filtration.