Abstract: A vertical exterior weather seal for use in an adhesively glazed curtain wall system has opposing gasket members formed on either side of a central stem. The rear edge of the stem is received within a channel formed in the front wall of a frame member, and splines formed on the sides of the stem adjacent its rear edge engage the walls of the channel to retain the weather seal in place. A pair of curtain wall panels are butt-glazed to the front wall of the framing member on opposing sides of the weather seal, and the tubular gasket members on either side of the stem bear against the mutually facing lateral edges of the adjacent curtain wall panels to seal the joint between the panels. Because the weather seal requires only that a channel be provided on the front wall of the mullion, the seal is easily adapted to a wide variety of mullion configurations.