Abstract: A fishing reel and changeable cartridge spool that is especially useable with a fly fishing reel or a spinning reel which allows cartridges containing fishing lines of different weights and densities to be quickly and readily interchanged onto the spool of a single reel. The improved reel spool includes a system that permits attachment of a first and a second cartridge spool filled with a line. The removable cartridge spool is mounted on the first spool which includes a threaded end face that is threadably connected to the spool axle and spool end face. The device employs one spool, which can be threadably separated and a cartridge spool which contains the line and which fits over the first spool axle and includes a locking detent so that the first and cartridge spools are in effect locked together. The first spool fits on the conventional reel shaft. The reel housing and the drag and winding mechanisms are not changed.