Abstract: A separator for separating a first from a second phase of a liquid in a tubular container includes a float made of elastic material having a circumferential sealing edge and at least one ballast fastened to the underside of the float. The density of the ballast is greater than the density of the float and the density of the entire separator lies in a value range between the density of the first phase and the density of the second phase of the liquid. In order to ensure an unrestricted light inflow of the liquid into volume regions of the container lying below the separator in the initial position, the float is designed disk-shaped; and that the ballast is designed in the form of a plurality of fingers extending away from the underside of the disk-shaped float, distributed at its edge.