Marsh Hoppers are small, frog-like aliens originating from the planet Rigil Kentaurus. When it spots an enemy, it will hop towards the enemy until they get close enough, where it will jump on the enemy and explode, releasing toxic slime that is stored in its body.
In-game, Hoppers have very low health, making them extremely easy to kill. However, Hoppers are always released in large hordes. This makes them very dangerous, as they can quickly jump on the player in succession and inflict a lot of damage in a very short amount of time. In higher difficulties, a group of Hoppers jumping on the player almost at once can quickly kill a player with 80%+ health and no armor.
- When fighting Marsh Hoppers, the player must always be on the move. Hoppers can quickly catch up to the player if they stop moving.
- The player must always be aware of their surroundings when fighting off Marsh Hoppers. Hoppers can easily sneak up on the player because their sounds are not location-based and will always sound the same, whether the Hopper is behind or in front of the player.
- Groups can be dealt with by using either the M1A2 Thompson or the Rocket Launcher. Smaller groups can be taken out with dual Schofields. The Rocket Launcher is effective because Marsh Hoppers easily stick together in tight groups, which makes them vulnerable to a rocket's splash damage. However, the player should only fire the Rocket Launcher at a distance because there is a chance of a Hopper jumping in front of a rocket at the last moment if fired close to Hoppers.
- It may be tempting to use the XM214-A Minigun against large hoards of Hoppers, it is actually a waste of ammo. It takes one bullet to kill a Hopper, and the Minigun's high rate of fire will ensure that a few rounds will be wasted while switching to another Hopper.
- While Hoppers may explode when they die, the explosion they make if they die does almost no damage, even at point blank range. Their explosions only do damage if they hop onto the player.
- In the Second Encounter, the Chainsaw can make a quick work of even a large marsh hopper group. When no other enemies are present, it's often a good idea to retreat in the corner and point the chainsaw diagonally upward, making it very hard to get hit.
- With other enemies, Marsh Hoppers are a medium-priority threat. Their attacks can be dodged but it may sometimes prove difficult.
- It may be useful to crouch while fighting Marsh Hoppers. While crouching, Marsh Hoppers are at your eye level and you don't have to angle your weapon down to hit them.