A ball valve is provided in which an outer housing (1) can be left in a pipeline during servicing and a sub-assembly can be removed as a unit from the top of the outer housing (1). The sub-assembly comprises an inner housing (8) in which a ball member 17 is journalled for rotation, and annular seal adaptors (13) for interposition between the ball member (17) and diametrically opposite openings (3) in the outer housing (1). The seal adaptors (13) are such that they can be easily withdrawn outwardly from the withdrawn sub-assembly to reveal the ball member 17 and the seal adaptors for inspection without having to remove the ball (17) itself. The seal adaptors (13) are also wedged-shaped so that when the sub-assembly is refitted in the outer housing (1), the seal adaptors (13) automatically come into sealing engagement with the ball member (17) and the outer housing (1).