A three-dimensional, primitive equation model is used to simulate the flow through a narrow strait. The flow within a strait has a simple two-layered structure with the interface located approximately at the middepth. However, the flow experiences a sharp transition at the strait's exit: the flow jumps from a subcritical velocity inside the strait to a supercritical velocity immediately outside the strait. Correspondingly, the density interface also slopes sharply upward to the surface. After exiting the strait the surface outflow tends to form an anticyclonic gyre to the right of the strait's exit. Most of the outflow water is trapped within the gyre, although some manages to escape through a narrow coastal current. These model results compare well with observations in the Tiran Strait and in the Gibraltar Strait and Alboran Sea. The model study indicates that the flow transition at the strait's exit and the formation of an anticyclonic gyre are unique features of the nonlinear self-advection of the density current. ¿American Geophysical Union 1987