The flamethrower is an experimental weapon that uses highly volatile napalm fuel to deliver a jet stream of flesh-penetrating flame. It is very effective for use on large masses, since the stream will engulf even partially hidden enemies. The fact that enemies will burn for a few moments after they've been hit makes this weapon even more effective. In addition, any enemy that touches it will be set on fire, which will do extra damage to them. The flame from the flamethrower can pierce multiple enemies. However, the flame from the flamethrower has limited range, making it useful only for short range combat.
The flamethrower is extremely useful against Kleers, as it quickly does damage to them; the burning effect it has can kill or severely damage a Kleer if it managed to survive the fire from the weapon, and more importantly, it can hit multiple Kleers at once. These attributes let the player quickly decimate a small to medium-sized group of Kleers, and cause considerable damage to larger hordes of them. The flamethrower should be the player's main anti-Kleer weapon once obtained. Cucurbitos are also a great target for the flamethrower.
The flamethrower is also handy against Marsh Hoppers, as it can kill any Hoppers that try to jump in front of the player without it harming the player with its suicidal attack. This makes it quite handy in areas such as the later part of The Citadel, where Kleers and Marsh Hoppers will spawn together.
Groups of other weak enemies, such as Beheaded Rocketeers and Gnaars, can be completely destroyed by this weapon. This makes it useful when a large amount of weak enemies appear in close quarters, such as the end of Ziggurat or Land of the Damned. It also makes fights against huge masses of Beheaded Kamikazes notably easier in the same way, without needing to use other long ranged rapid-firing weapons.
The flamethrower is not effective against strong enemies, such as the Major Bio-mechanoid or the Common Aludran Reptiloid. It is not damaging enough to quickly harm them and the weapon's range puts the player too close to them, giving the player less room to dodge than they would with other weapons.
Lava Golems are completely immune to this weapon, therefore the flamethrower should not be used on them.
The flamethrower was originally going to be in Serious Sam: The First Encounter, but was removed before the game was released. An icon for its ammo can still be found in the game's files, and coding for it and its ammo can be found in The First Encounter's SDK. Early screenshots for The Second Encounter show that The Second Encounter flamethrower used the unused First Encounter icon for it during development, but it was replaced in the final game.
If a Gnaar is killed with the flamethrower in Serious Sam HD, their corpse may occasionally stay standing up and show the Gnaar in its idle pose. Their corpse cannot be gibbed with any weapons or explosives and will not disappear unless the player loads their last save point or completes the level. This is fixed in Serious Sam Fusion 2017.