A ball valve assembly comprises a housing with ball valve rotatably disposed in a valve chamber in the flow passage. The ball valve is rotatable between open and closed positions by a valve stem that is connected to the ball valve at one end. The ball valve has an open ended slot in which the end of the stem is received. During assembly the stem may be inserted into the housing first and then the ball valve introduced into the housing with the slot facing the stem and then moved in the direction of the slot so that that the stem is received in the slot. The arrangement enables the conventional bonnet in a ball valve assembly to be eliminated and therefore provides for a compact design.