The mature version of a Miniroid, the Phazon Metroid is similar in appearance to Metroids, with the differences being that a bony ridge splits their membrane down the middle, their larger talons are sharper and stick out more, and they have what appears to be a vestigial fifth claw on the back. Phazon mutation has also given them a tiny mouth located on their underside that has a small ring of smaller teeth surrounding it. If one latches onto Samus, she must go into Morph Ball form and lay a Bomb to get it off. Unlike normal Metroids, Phazon Metroids have the strength to carry the Morph Ball into the air, limiting the effects of ill-timed Bombs and increasing their threat level. She can use the Hyper Ball as an alternative to escape their grip and possibly kill them if they remain long enough in close proximity. In addition to their other abilities, they can temporarily warp out of local spacetime, making them intangible. They'll often do this to avoid being frozen by an Ice Missile, since they share the Metroids' weakness to cold; they cannot do so, however, when preparing to fly at a victim. They also cannot turn intangible if Samus shoots at them with her beams, as they travel much quicker than a Missile, and Hyper Mode can be used to destroy them quickly and effectively. A fully-charged Hyper Beam will destroy a Phazon Metroid instantly. When a Phazon Metroid leeches energy, it does not increase in size like previous Metroid variants in the Prime games.
Their membranes have properties similar to Phazite, and cover a weak point that can be targeted with the X-Ray Visor and destroyed with the Nova Beam for a quick kill. Another unnatural trait the creatures possess is the capacity to fire a bolt of phazon energy that resembles an electric arc to engage foes at a distance, making them very dangerous opponents at long-range as well. These bolts can cause a Hyper Mode failure.
The Phazon Metroids appear mostly in Space Pirate areas and on Phaaze. They can also be spawned by a Metroid Hatcher. The creatures known as Hopping Metroids are most likely a higher form of Phazon Metroids, as they are described as Metroids radically mutated by Phazon.
Phazon Metroids seem to have the capacity to make hives, as seen on Phaaze where they contantly respawn from structures in the walls positively identifed as Metroid hives in scans.
"Like all Metroids, great cold can immobilize the Phazon Metroid and leave it vulnerable to attacks. If the creature begins to feed, a concussive blast should be able to knock it away. Heavy exposure to Phazon energy has mutated this strain of Tallon Metroid. Subject can shift its body into a phased state, allowing it to ignore solid matter. The creature can emit bursts of Phazon energy, and will drain the life energy of its prey."
When in the Xenoresearch Labs in SkyTown, and while the Phazon Metroids are still in their containers, one may notice that they have varying "personalities". While some will instantly go into attack position as Samus walks by their tanks, others merely chirp as they sway side to side or even simply ignore Samus.
After aquiring the Seeker Missiles, some Metroids can be seen phasing through the walls to attack Samus, but then immediately rushing back.
The back claws of a Phazon Metroid resemble the tips of the second legs on Metroid Prime's first form, but their overall appearance (their shape and nuclei) shares more similarities with the same creature's core form, perhaps hinting at the possible connection between Phazon Metroids and Metroid Prime.
The vague appearance of a humanoid face is possible to make out on the frontal membrane of the Metroid: eyes, a nose and possibly a mouth are visible. The Metroid Prime's first and second forms had humanoid faces as well.
Another trait that Phazon Metroids share with the Metroid Prime's second form is the ability to phase out of the local timespace, becoming invulnerable to all weapons except those of Phazon origin.
Oddly enough, they ignore Space Pirates and vice versa when they're in the same room and attack Samus in tandem, unlike in Metroid Prime and Metroid Prime 2: Echoes. Most likely they were never scripted to encounter each other during programming, so no exceptional attacks occured.
Phazon Metroids can possibly survive in space as seen in the Valhalla, in which the predators can be witnessed floating through space; some enter the ship while phasing out of local timespace. The mentioned creatures seen outside out of the ship may, however, be dead Metroids (as they show no movements) and those that enter into the vessel could be from adjacent rooms. However, it does not seem possible that the Metroids could have died outside the ship, as the Valhalla battle was mostly inside the ship and Metroids do not seem to require oxygen to survive. Further proof of this is in Metroid Fusion, where an Omega Metroid was in the hanger bay of the Biologic Space Laboratories research station with a broken door showing the vacuum of space, possibly having the room decompressing.
Phazon Metroids on Phaaze are shown living within hives from which the creatures are constantly spawned. Also, an incredible amount of corrupted Metroid Eggs are present. The requirements necessary for a Phazon Metroid hive to be created were never revealed.