A single-lever mixer tap, which comprises: - at least one housing with a first inlet of hot liquid, a second inlet of cold liquid, and several outlets (13, 14, 15) of liquid; - a mixing cartridge in order to mix the cold and hot liquids; where said mixing cartridge sits on a bottom with a water passage that lets hot water pass to the mixing cartridge and a water passage that lets cold water pass to the mixing cartridge; - a selector cartridge to select one of the outlets (13, 14, 15); - the at least one housing comprises a one-piece body including at least a first head for mixing liquids, where said first head is coupled to the mixing cartridge; and where said first head comprises a first chamber where the first inlet leads into, a second chamber where the second inlet leads into, and a third chamber connected to a common outlet for the liquid in the mixing cartridge; - the third chamber of the first head is connected to an intermediate chamber of the housing; - the housing also comprises a second selector head to send the flow of water towards one of the outlets (13, 14, 15); where said second selector head is connected to the selector cartridge; and where the intermediate chamber is connected to a space enclosing the selector cartridge coupled to the second head; - the selector cartridge sits on a bottom that includes several holes (13', 14', 15') that are connected independently with the outlets (13, 14, 15); wherein - the first head of the housing comprises a first box that comprises the bottom and a first annular part that has an inner surface on which a tubular-shaped guide screw is fixed, which acts as a guiding component of the mixing cartridge held axially by means of a first fixing screw that screws in the guide screw; - the second head of the housing comprises a second box that comprises the bottom and a second annular part that has an inner surface where a second fixing screw is fixed, which axially holds the selector cartridge; the single-lever mixer tap further comprises a casing formed by a bottom and an encasing lateral wall; where said bottom comprises a first opening and a second opening on which a series of outer encasing surfaces of the first and second annular parts (5a, 11a) of the first and second boxes (5, 11) are secured. - at least one housing with a first inlet of hot liquid, a second inlet of cold liquid, and several outlets (13, 14, 15) of liquid; - a mixing cartridge in order to mix the cold and hot liquids; where said mixing cartridge sits on a bottom with a water passage that lets hot water pass to the mixing cartridge and a water passage that lets cold water pass to the mixing cartridge; - a selector cartridge to select one of the outlets (13, 14, 15); - the at least one housing comprises a one-piece body including at least a first head for mixing liquids, where said first head is coupled to the mixing cartridge; and where said first head comprises a first chamber where the first inlet leads into, a second chamber where the second inlet leads into, and a third chamber connected to a common outlet for the liquid in the mixing cartridge; - the third chamber of the first head is connected to an intermediate chamber of the housing; - the housing also comprises a second selector head to send the flow of water towards one of the outlets (13, 14, 15); where said second selector head is connected to the selector cartridge; and where the intermediate chamber is connected to a space enclosing the selector cartridge coupled to the second head; - the selector cartridge sits on a bottom that includes several holes (13', 14', 15') that are connected independently with the outlets (13, 14, 15); wherein - the first head of the housing comprises a first box that comprises the bottom and a first annular part that has an inner surface on which a tubular-shaped guide screw is fixed, which acts as a guiding component of the mixing cartridge held axially by means of a first fixing screw that screws in the guide screw; - the second head of the housing comprises a second box that comprises the bottom and a second annular part that has an inner surface where a second fixing screw is fixed, which axially holds the selector cartridge;