Abstract: An electromagnetic interference (EMI) and environmentally protected cap assembly for electrical connectors, for use when the mating connector is not in place, includes a hollow outer housing having a large diameter rear end and a smaller diameter front end having inwardly directed teeth for locking behind the outwardly extending lugs on the cylindrical form of the connector. An inner body member which is rotatable with respect to the housing, is spring biased by a plurality of wave washers toward the front of the outer housing. The inner housing has recesses which mate with the lugs on the connector to prevent relative rotation of these two parts, and detenting means are provided between the housing and the inner body member for locking the cap assembly onto the connector when the outer housing is assembled to the connector and rotated.