Cathode unit for installation in a fluorescent tube body (3) belonging to a fluorescent tube (1), which cathode unit (5) comprises a cathode screen (15a, 15, 15'-15'''') which partially surrounds an electrode (9) which is electrically insulated from the said cathode screen (15), a power supply device (11) arranged to make an electrical connection between the said electrode (9) and a contact (13), the said cathode screen (15a, 15 , 15'-15'''') comprising a first end (19) facing towards the discharge, which first end (19) comprises a central opening (21), and a second end (39) facing towards the said contact (13). The first end (19) of the cathode screen (15a, 15, 15'-15'''') is designed with a rounded-off part (25) in order to facilitate the insertion of the cathode unit (5) in the said fluorescent tube body (3).