A wellhead sealing system comprises a casing (12), a tubing spool (16) and a bushing (22) between the casing (12) and the tubing spool (16). A first recess (24) is provided in the bushing (22) and a first casing seal (23) is mounted therein to act between the bushing (22) and the casing (12). A second recess (26) is provided in the tubing spool (16) and a first bushing seal (25) is mounted therein to act between the tubing spool (16) and the bushing (22). The casing seal (23) has cylindrical inside (27) and outside (28) surfaces located generally at the inside and outside diameters of the seal (23). The cylindrical inside surface (27) is defined by a radially inwardly extending convex central protuberance (29) and opposed cylindrical flat surface areas (30) extending axially outwardly from the convex central protuberance (29) into first transition areas (31) extending to respective end surfaces (32). The cylindrical outside surface (28) is defined by a circumferential centrally located groove (33) flanked by axially outwardly extending flexible retaining flanges (34) forming second transition areas between the cylindrical outside surface (28) and the end surfaces (32). Spring means (35) extend around the inside circumference of the seal and are located adjacent each of the first transition areas (31).