On a round plug, outwardly directed detent hooks are arranged between a plug-in region and a contact carrier portion of an insulating body or a base portion of the round plug housing. A mating connector has, on a plug-in side of a threaded portion, an inwardly directed circumferential or interrupted detent lug. The detent arms do not need to reach through the threaded portion. The detent arms can therefore be made particularly strong, and the inner thread can be continuous. The retaining forces of the locking/screwing are thus very high, and the detent arms for example can be integrally molded on a plastic insulation body, and thus be formed in one piece therewith, for simplified production. Operation and manufacture are significantly simplified.