Writing implement comprising: - a barrel (1; 10) extending in a longitudinal direction; - a writing cartridge (2; 20) provided at one front end with a tip (2A; 20A) and designed to be moved in the barrel in said longitudinal direction between a forward position in which the tip (2A; 20A) is extended and a retracted position in which the tip is retracted into the barrel; - a push member (3; 3', 3"; 30) movable in the longitudinal direction and in a radial direction relative to the barrel, connected to the cartridge in order to move the latter, comprising at least one control member (3A; 30A) extending through an opening (4; 40) in the side of the barrel in order to project radially outwards, and also comprising at least one locking member (3B; 30B) designed to cooperate with a complementary locking member (5; 51) of the barrel allowing the push member to be locked in the forward position of the cartridge; characterized in that the push member (3; 3'; 3"; 30) is directly fitted on a flexible tubular part (2B; 20B) of the cartridge (2; 20), said flexible tubular part (2B; 20B) of the cartridge (2; 20) being movable jointly with the push member (3; 3'; 3"; 30) and forming an elastic return member designed to exert on the push member a force directed outwards from the barrel (1; 10) making it possible to bring about said locking, and the writing implement comprising a front radial adjustment means (3D; 90) and a rear radial adjustment means (9; 90) of the cartridge arranged in the barrel on either side of the position of said control member (3A; 30A) and designed to limit at their respective levels the movement of said flexible tubular part (2B; 20B) in the radial direction during the movement of the push member (3; 3"; 30). - a barrel (1; 10) extending in a longitudinal direction; - a writing cartridge (2; 20) provided at one front end with a tip (2A; 20A) and designed to be moved in the barrel in said longitudinal direction between a forward position in which the tip (2A; 20A) is extended and a retracted position in which the tip is retracted into the barrel; - a push member (3; 3', 3"; 30) movable in the longitudinal direction and in a radial direction relative to the barrel, connected to the cartridge in order to move the latter, comprising at least one control member (3A; 30A) extending through an opening (4; 40) in the side of the barrel in order to project radially outwards, and also comprising at least one locking member (3B; 30B) designed to cooperate with a complementary locking member (5; 51) of the barrel allowing the push member to be locked in the forward position of the cartridge; characterized in that the push member (3; 3'; 3"; 30) is directly fitted on a flexible tubular part (2B; 20B) of the cartridge (2; 20), said flexible tubular part (2B; 20B) of the cartridge (2; 20) being movable jointly with the push member (3; 3'; 3"; 30) and forming an elastic return member designed to exert on the push member a force directed outwards from the barrel (1; 10) making it possible to bring about said locking, and the writing implement comprising a front radial adjustment means (3D; 90) and a rear radial adjustment means (9; 90) of the cartridge arranged in the barrel on either side of the position of said control member (3A; 30A) and designed to limit at their respective levels the movement of said flexible tubular part (2B; 20B) in the radial direction during the movement of the push member (3; 3"; 30).