Successful numerical models of faults, joints, and other fractures in a rock mass require specification of appropriate boundary conditions, but the choice may not be obvious. To illustrate potential pitfalls, a single two-dimensional fracture is modeled by the displacement discontinuity method of boundary elements using both displacement boundary conditions and traction boundary conditions. The results are compared with the analytical solution for a mode I (opening) crack. It is shown that the traction boundary conditions can provide a satisfactory solution, while displacement boundary conditions, applied to a problem with both continuous and discontinuous displacement components, can introduce significant errors. ¿ American Geophysical Union 1992