The problem of an interface crack between a circular fiber and the surrounding matrix is considered. The problem is formulated and solved with the help of complex variable methods. It is essential to take into account the existence of contact zones at the crack tips. The solution procedure relies on the use of crack opening displacements as the primary variables. Ultimately the governing equations are shown to consist of two coupled singular integral equations together with contact and single valuedness conditions. In general these equations must be solved by numerical methods. Attention is focused on the lengths of the contact zones. It is shown that the lengths of these contact zones are essentially independent of one of the Dundurs parameters.