An analysis of a crack lying along an interface between two elastically dissimilar quarter-planes, and breaking the free surface is given. The method of distributed dislocations is employed and the nominal stress field is taken to be uniform along the length of the line of the crack. Models for crack tip behaviour are discussed and it is shown that a simplified quadrature can be used to extract the crack extension force for cases where small-scale contact obtains. Values of the crack extension force are given, displayed on the α, β diagram.