A filling unit for filling containers with a liquid substance, comprising: - a plate forming a resting surface for the base in a container to be filled; - a housing configured to receive a mouth of the container to be filled, the housing facing the plate and forming a cavity which opens towards the plate; - a dispenser which opens onto the housing and faces the plate, the dispenser - in use - facing the mouth of the container so as to dispense, through a dispensing opening, the liquid substance towards an internal volume of the container; - a feed duct for feeding the liquid substance to the dispenser; - a valve interposed between the feed duct and the dispensing opening, the valve being controllable to assume an open position and a closed position, thus allowing and preventing the liquid substance to be dispensed from the dispenser; the filling unit being configured to fill containers of a first type and to fill containers of a second type, the housing being configured to receive the mouth of a container of the first type and, alternatively, the mouth of a container of a second type, wherein the housing comprises a bottom wall, on which the dispenser opens, and a lateral wall, the cavity of the housing being delimited by the bottom wall and the lateral wall, the housing being fitted with a first annular gasket, designed to come into contact with the mouth of a container of the first type, and a second annular gasket, designed to come into contact with the mouth of a container of the second type, wherein the first annular gasket is smaller in size than the second annular gasket and is positioned at a shorter distance from the bottom wall of the housing, the lateral wall of the housing being formed at least partly by an annular body that is movable relative to the bottom wall, moving towards or, vice versa, away from the plate, the second annular gasket being mounted on said annular body, characterised in that the plate is movable towards or, vice versa, away from the dispenser, and in that the filling unit is configured to operate in such a way that: - after a container of the first type has been positioned on the plate, the plate is moved towards the dispenser to bring the mouth of the container into contact with the first annular gasket; and - after a container of the second type has been positioned on the plate, the annular body of the housing is moved towards the plate to bring the second annular gasket into contact with the mouth of the container. - a plate forming a resting surface for the base in a container to be filled; - a housing configured to receive a mouth of the container to be filled, the housing facing the plate and forming a cavity which opens towards the plate; - a dispenser which opens onto the housing and faces the plate, the dispenser - in use - facing the mouth of the container so as to dispense, through a dispensing opening, the liquid substance towards an internal volume of the container; - a feed duct for feeding the liquid substance to the dispenser; - a valve interposed between the feed duct and the dispensing opening, the valve being controllable to assume an open position and a closed position, thus allowing and preventing the liquid substance to be dispensed from the dispenser; the filling unit being configured to fill containers of a first type and to fill containers of a second type, the housing being configured to receive the mouth of a container of the first type and, alternatively, the mouth of a container of a second type, wherein the housing comprises a bottom wall, on which the dispenser opens, and a lateral wall, the cavity of the housing being delimited by the bottom wall and the lateral wall, the housing being fitted with a first annular gasket, designed to come into contact with the mouth of a container of the first type, and a second annular gasket, designed to come into contact with the mouth of a container of the second type, wherein the first annular gasket is smaller in size than the second annular gasket and is positioned at a shorter distance from the bottom wall of the housing, the lateral wall of the housing being formed at least partly by an annular body that is movable relative to the bottom wall, moving towards or, vice versa, away from the plate, the second annular gasket being mounted on said annular body, characterised in that the plate is movable towards or, vice versa, away from the dispenser, and in that the filling unit is configured to operate in such a way that: - after a container of the first type has been positioned on the plate, the plate is moved towards the dispenser to bring the mouth of the container into contact with the first annular gasket; and - after a container of the second type has been positioned on the plate, the annular body of the housing is moved towards the plate to bring the second annular gasket into contact with the mouth of the container. - after a container of the first type has been positioned on the plate, the plate is moved towards the dispenser to bring the mouth of the container into contact with the first annular gasket; and - after a container of the second type has been positioned on the plate, the annular body of the housing is moved towards the plate to bring the second annular gasket into contact with the mouth of the container.