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Shroob UFOs are the Shroob's alien spaceships that they use for travel from place to place in Mario & Luigi: Partners in Time. These spaceships are small, but they can fire powerful energy bolts that can turn people into Shroob Mushrooms, as well as deal devastating damage to buildings and such.
In Hollijolli Village, Mario and Luigi have to fight an impossible battle that featured a Shroob UFO that counted down from three. When it reached zero, Mario and Luigi were defeated. Much later in the game, the player battles them along with the Shroob Mothership. The same kind of UFO also appears in the second half of the Elder Shrooboid battle, but can be hit with a Spiked Ball to reset the countdown. Shroob UFOs also appear in battles with Blazing Shroobs, where they lower fireworks down to the Blazing Shroob's flames, and shoot at the Bros. with it. They can be knocked out of battle by jumping onto one of the Blazing Shroobs while they're aflame.
Shroob UFOs are powered by Toad energy, known as vim. The vim is sucked out of the Toads by the trees of Toadwood Forest mixed with chemicals at the Vim Factory, and afterward used as fuel for the Shroob UFOs. They are summoned using a device that strongly resembles a Nintendo DS.