Abstract: A seal for a duct (7) containing a transmission line package (P) comprises a flexible member (2) disposed in a connector (1). The flexible member (2) has a waisted portion that collapses inwardly onto the transmission line package (P) to form a gas seal up to pressures of about 2 bar, above which the waisted portion yields to permit removal or insertion of the transmission line package, for example by the fibre blowing technique. Vents (8) are provided in the connector (1) so that an increase in ambient pressure externally of the connector increases the resistance of the waisted portion to opening. The arrangement may be used to maintain water tightness or pressure differentials over a bulkhead.