Abstract: The plug is disposable around a fiber optic cable in an innerduct to form the present assembly. The plug releasably seals the cable and innerduct against ingress of mud, dirt, water, grit and other foreign substances. The plug includes upper and lower rigid compression plates positioned, respectively, immediately above and below a flexible, resilient tubular gasket having an upper circular flange. The lower compression plate has two or more spaced vertical curved arms disposed in a circle around a central vertical cable-receiving passageway and extending up from the plate through slots in the gasket and upper compression plate. The arms are threaded externally and hae an internally threaded collar screwed down thereover. The collar, upper plate and flange seat over theopen end of an innerduct bearing the cable. the lower compression plate and gasket (except for the flange) are in the innerduct. The gasket, upper and lower compression plates and collar have aligned cable-receiving vertical passageways.