Abstract: An ankle joint prosthesis comprises an upper part for attachment to an artificial leg and a lower part for attachment to an artificial foot, with the upper and lower parts rotatively coupled and capable of being fixed in a first position for walking, a second position for swimming, or a free-flexing mode for activities such as skiing or rowing. The upper and lower ankle parts have coaxial bores in which a cam part is mounted by means of two bushings. A pin-and-slot arrangement allows locking of the prosthesis in a desired position. Changing positions takes only seconds, and is effected by rotating a spring-loaded "D"-ring on the side of the ankle prosthesis by hand. In a preferred embodiment the ankle joint prosthesis is completely waterproof and precision machined from a lightweight, high-strength plastic.