Samus encounters them in the Phazon Core of the Impact Crater, Metroid Prime is also capable of spawning them. In the PAL and Metroid Prime: Trilogy version of the game, they can also be found in the bottom level of the Phazon Mines after the Omega Pirate is defeated. These creatures have specific vulnerabilities to different elements of nature, and therefore different one of Samus's Chozo-based beam weapons.
Fission Metroids are capable of spitting into two Metroids, each vulnerable to a specific beam weapons. Due to the chaotic nature of Phazon, this vulnerability is completely random. Before splitting, Samus may damage a Fission Metroid with any beam weapon; after splitting, she may damage them with the beam corresponding to its color as well as the Charge Combo.
- Yellow: Power Beam and Super Missile
- Purple: Wave Beam and Wavebuster
- White: Ice Beam and Ice Spreader
- Red: Plasma Beam and Flamethrower
- "The Fission Metroid is a mutant, capable of splitting in two. This split endows the new creatures with invulnerability to most weaponry. This effect is unstable, resulting in weakness to a type of weapon fire. The vulnerability appears to be random, due to the chaotic nature of Phazon mutation." — Logbook "Fission Metroid" (Metroid Prime)