The invention provides a plug for sealing an end of a duct. The plug includes a sealing member for sealing against an interior surface of the duct, and gripping means having one or more teeth for gripping the interior surface of the duct. The plug may include a pair of opposed camming surfaces arranged for relative axial movement therebetween whereby the relative axial movement results in the teeth of the gripping means being urged radially outwardly into gripping engagement with the interior surface of the duct.