For the Boss version of Rootmars, see Rootmars.
At the end of Mission 2, Morden explains to Marco and his team what happened several weeks ago. After the events of Metal Slug 3, Rootmars slowly recovered itself from the depths of the ocean. As Rootmars lay dormant under the sea, the Martians were left defenseless against their natural predators, the Invaders, who took the opportunity to conquer both the Martians and Earth. With the help of the Rebel Army, the Martians rescued Rootmars by picking it up from under a bridge. Despite the Invader's attacking the rescue site dead on, Rootmars fully recovers and helps out the triple alliance.
In Mission 4, near the end of the first area, below on the highway, the sound of Rootmars' cackles is heard. It dives down grabs the players, taking the role of a Slug (the upper road has Slug Flyer instead). It has a 360-degree 10mm Vulcan and an electric brainwave (usable via the Bomb/Cannon Button) that covers the entire screen, but doesn't offer protection to the players. To avoid damage during the ride, the players change the position of Rootmars' arm they are holding on to by moving up and down on the joystick.
For Rootmars' Vulcans, Player 1 controls the right Vulcan while Player 2 controls the left Vulcan. It also comes with a built-in Vulcan Fix which locks down the position of the controlled Vulcan when holding down the fire button.
Graphically, Rootmars has a dome-like helmet to protect its only weak spot, and it no longer has any stitches on its face.
In Other Games
Rootmars Slug appears as part of the Regular Army faction. It is the second toughest unit with 30,000 HP at level 30. It is also the most expensive unit to buy and deploy. Rootmars cost 500 medals and it needs 999 AP to be deployed in battle.
Rootmars returns as a "Pack Shop Unit" for the New Year's Celebration. Its base AP cost (on iron tier) is the highest to date, being 3000 AP (1000 AP on Platinum). Rootmars also has a counterattack move. Parts of the Rootmars Slug are now available from the Mars Shop.
- Prior to Metal Slug Infinity, Rootmars was one of the two bosses that could be used as Slugs (the other being Black Hound, however, Black Hound is a mid boss, rather than a level-end boss)
- It is unknown as to how Rootmars managed to recover from its injuries after its defeat in Metal Slug 3, as Rootmars can be seen on the credit's background severely disfigured, having lost an arm, a few teeth and part of its brain.
- In Metal Slug Attack, Normal Rootmars reuses Rootmars Slug's sprite, albeit without the Vulcans nor the glass dome. As a result, Rootmars has two "horns" on his forehead.