In the dung flyCoproica vagansHaliday 1833 (Diptera, Sphaeroceridae), a firm but elastic mating plug blocks the female genital opening after copulation. The plug does not contain sperm. Sperm are present in the form of a mass that fills the vagina anterior to the mating plug. Experiments using males marked with acridine orange demonstrated that the mating plug is produced by males. Copulations always resulted in plug formation. Male remating intervals seem to be adjusted to the time needed to replenish the material needed for mating plug production. Two non-exclusive functions of the mating plug are proposed: retention of sperm in the vagina after copulation and paternity assurance by preventing females mechanically from remating until oviposition. Shape and position of theC. vagansmating plug provide evidence for a capability of retaining sperm inside the vagina until sperm reaches the female storage organs. The mating plug prevents remating while it is in the vagina. Following copulationC. vagansfemales are not sexually available to other males. Mechanical manipulation of females seems to be the main cause for this post-mating loss of ability to copulate. Mating plugs persist unaltered until oviposition begins. Males were not able to induce plug expulsion or remove plugs with their genitalia. The mating plug is pushed out when the first egg is laid. Eggs are deposited in batches. Remating occurred on days when an egg batch laid by that female was observed, never on following days. In the field 96% ofC. vagansfemales carry a mating plug. Copulation duration is fairly constant and is not correlated with female mating status.