Abstract: A butterfly valve comprising a body 1, a passage 2 extending through the body, a closure or butterfly member 3 mounted in the passage 2 to pivot about an axis orthogonal to the axis of the passage so as to be movable between a closing position in which a sealing element fixed or formed on the periphery of the disc of the closure member has a sealing contact with a seat 8 carried by the body and a fully open position roughly perpendicular to the closing position, the peripheral surface of the closure member 3 and the conjugate surface of the seat 8 being conical surfaces defined by two cones of revolution, while the axis of rotation of the closing member is offset along the axis of the passage in a direction away from the apex of the cone defining the surface of the seat relative to the mean plane of the disc of the closing member, wherein for a given value of the apex angle 2.phi. of the cone C.sub.