To study the stability of mode I (opening-mode) fracture, we consider a two-dimensional system in which a crack moves along the center line of a very wide, infinitely long strip. We compute the first-order response of the crack to a spatially periodic, perturbing shear stress. We assume isotropic linear elasticity in the strip and a cohesive-zone model of the crack tip. The behavior of this system is strongly sensitive to the dynamics within the cohesive zone; stability cannot be deduced simply from properties of the far-field stress-intensity factors. When the mode I and mode II (sliding-mode) fracture energies are equal, the crack is marginally stable at zero speed and is unstable against deflection at all nonzero speeds. However, when the cohesive stress has a shear component that strongly resists bending into mode II, there is a nonvanishing critical velocity for the onset of instability. \textcopyright{} 1996 The American Physical Society.