Abstract: A ball valve made entirely of plastics material comprising a tubular casing with connection pieces which are fastened to both its ends and which together form a through passage for a medium, a ball disposed in the through passage inside the casing and provided with a through opening, operating means formed by a spindle inserted into an opening in the casing, cooperating with the ball, and provided with an operating grip to enable the ball to be turned so as to close or open the through passage. The casing consists of a substantially cylindrical outer casing to each end of which the connection pieces are welded, and of an inner casing disposed therein. The inner casing being constructed of two parts each of which having a seat. Both parts are connected by means of a screwthread and enclose the ball in between them.