Mullite and zircon ceramic matrix composites uniaxially reinforced with as-supplied and BN-coated silicon carbide filaments were fabricated. The filament-matrix interfacial shear stresses were measured by a modified indentation technique and mechanical properties were determined in flexure mode. Significant improvements in strength and toughness were observed for the composites in comparison to monolithic mullite and zircon ceramics. Lower filament-matrix interfacial shear stresses were measured in composites with BN-coated filaments than in composites containing as-supplied filaments. The influence of filament-matrix interfacial shear stress on the critical stress for first matrix cracking, maximum composite strength, and toughness of the composites were studied.