A method is disclosed of forming a bag having a bottom seal from a web of sealable sheet material which is formed into a tube (12) by sealing opposite longitudinal edge portions of the sheet material one to another. The method comprises feeding the tube (12) in flattened form further along a feed path to a severing station (10), severing the flattened tube (12) in passage through the severing station (10) into individual flattened bag lengths (36) by passage between a rotary knife (26) and a counter blade (26'), feeding the individual flattened bag lengths (36) longitudinally along the feed path to a sealing station (11), and applying pressure, and possibly also heat, to opposite faces of an end portion of each flattened bag length (36) as it passes through the sealing station (11) by means of blocks (31) for a predetermined time which is longer than the period between severing that bag length (36) from the flattened tube (12) and severing the next bag length (36) from the flattened tube (12) while continuing to feed the flattened bag length (36) longitudinally along the feed path thereby to form a bottom seal (41) for the bag. Preferably the flattened tube (12) is formed with gussets (14, 15). The blocks (31) can be heated if the web is made of a heat sealable material. The heater bars (38, 39) for the gussets (14, 15) can be mounted on a different, more lightly spring loaded, mounting block from that for the heater bar (37) which forms the transverse bottom seal (41). By using rollers (24, 25) and making blocks (31) move slightly faster than the speed of the flattened tube (12) prior to severance of the bag length (36) it can be ensured that the bag lengths (36) are positively severed from the flattened tube (12) despite possible wear of, or damage to, the cutter blades (26, 26'). Registration of the printing on the web (12) so that the blocks (31) contact the web (12) in appropriate position to form the bottom seal (41) can be achieved, when the desired registration is disturbed, by temporarily varying the feed speed of the web (12) with respect to the speed of rotation of the rotary knife (26) so as to vary temporarily the lengths of the bag lengths (36) severed from the tube (12) until the desired registration is re-established.