The invention relates to an adapter for accommodating electrical installation devices and for fixing to a conductor rail system, comprising a housing which is made of plastics material and comprises an upper housing wall on which means for mounting an electrical installation device are provided, comprising feet for fixing to the conductor rail system by gripping, comprising a first contact member for connecting the electrical installation device, the first contact member extending at least substantially within the housing, and comprising a second contact member, electrically connected to the first contact member, for contacting a respective one of the conductor rails of a conductor rail system, characterised in that the housing comprises a guide recess, the second contact member being provided so as to be displaceable along said recess within the housing, in that the second contact member comprises a guide means and catch means, and in that catch recesses, with which the catch means of the second contact member can be brought into engagement, are provided in predetermined catch positions of the housing.