'Sepals/Perianth' was an Allied operation by the Special Operations Executive and Office of Strategic Services to establish the clandestine 'Sepals' and 'Sepals I' bases, masquerading as intelligence-gathering stations, in neutral Sweden opposite Narvik in German-occupied Norway for operations across the border (August 1944/May 1945).

From these bases the 'Perianth', 'Perianth I' and 'Perianth II' operating units, under the command of Lieutenant Håkon Kyllingmark, were to supply local intelligence, establish a radio link, and prepare attacks against targets in Norway, although nothing would be undertaken without authority from London. In November permission was given to extend the operation’s remit to the undertaking of sabotage efforts. By mid-September 1944 both bases were manned and had established radio contact, but for the rest of the year supplied only intelligence. In mid-January preparations were stated to the establishment of 'Sepals III' as a third base, whereupon the original two bases became 'Sepals I' and 'Sepals II'. In March 1945 another base was established as 'Sepals Gorgon'.

From mid-January the operating teams undertook sabotage missions, initially from 'Sepals I' but then primarily from 'Sepals III'. The bases were also links, sources of supplies, and organisational points for the Milorg military resistance organisation’s groups around Narvik and in the county of Troms.

The bases finally played an important role in the liberation of the area in May 1945 and in the protection of local sites.