A buckle for a seat belt, comprises a buckle housing (10) and two tongues (12, 14). The buckle housing (10, 90) contains two latching levers (40, 42) arranged for angular movement about respective spaced mutually parallel pivot axes (44, 46). The levers (40, 42) have hook formations (52, 54) which are shaped so that a force applied to a tongue (12, 14) in a direction to remove it from the buckle housing (10) tends to cause displacement of the corresponding lever (40, 42) from its locking position to its release position. Blocking means (80) is manually movable between a blocking position in which the levers (40, 42) are retained in their locked positions and a release position in which the levers (40, 42) are free to move between their locking positions and their release positions.