The invention relates to an adapter of the type where a coupling for glass fiber plug-in connectors is inserted from the front into a front plate and is locked by a spring. The inventive adapter comprises a one-piece or two-piece housing which comprises at least on one side a flange projecting from the cross-section of the housing against which the front plate rests, at least one plug-in connector recess which is disposed on the front end and/or rear end with respect to the front plate, and at least one form-fit detachable locking device for the plug-in connector. Every plug-in connector recess comprises an opening for alternately receiving at least two of three different plug-in connector types LX.5, LC and MU of and one respective locking device suitable for the respective type. The adapter can be configured as a simplex plug-in coupling for alternately receiving all three types, or as a duplex plug-in connector for alternately receiving the plug types LX.5 and LC. The inventive adapter further comprises a pin for actuating an opening flap of the plug type LX.5 on a displaceable element on a side wall of the adapter. The adapter can further be closed by a pivoted cap that is detachably mounted on the housing, and a catch spring, actuated from the front and disposed on the housing, that engages behind the front plate and releases the catch spring from the front plate.