An apparatus for use in sealing layers of thermoplastic material together from one side of the layers includes a heat sealing electrode having a first conductive element mounted on a support and adapted to be connected to a source of high frequency energy, and a second conductive element mounted on the same support but in spaced general parallel relation to the first conductive element and adapted to be connected to an electrical ground. The conductive elements are electrically insulated from each other and, when they are applied to one side of the thermoplastic sheet material to be sealed and high frequency energy is supplied to the first electrode, a seal is formed in the material beneath and between the ends of the electrode elements.