Measurements are reported of tear strength for several carbon-black-filled compounds over a wide range of temperature. Attention is focussed on “knotty tearing” when the tear deviates from the straight-ahead direction and sometimes splits, and the tear force rises markedly. Knotty tearing occurs only over certain temperature ranges which appear to be related to the glass temperature of the elastomer. It is suppressed when the crosslink concentration is either unusually high or unusually low. It is strongly dependent on the type of carbon black, and sensitive to the particular elastomer used, being virtually absent with one cis-polybutadiene elastomer although evident with others. It appears to be related to the ability of the compound to create a high degree of orientation of carbon black particles at the crack tip and thus induce a marked anisotropy of strength in this region. However, it was often absent in pure shear experiments, when the overall extension of the specimen was much smaller, and absent in conventional tensile tests when the initial crack was small and grew rapidly as a smooth fracture. Even in the absence of knotty tearing, the tear strength of filled compounds was found to be five to ten times higher than for the corresponding unfilled compounds. This is attributed, at least in part, to enhanced hysteresis.