The Flood Stalker Form is a crustacean-like Flood Pure Form that specializes in stealth and agility. It is apparently made from the re-casted bones of dead enemies. It will frequently mutate into either a Ranged Form or a Tank Form when the need arises. This form sits low beneath your field of view and their attacks can go widely unnoticed until you look down. They are able to crawl on walls and ceilings with ease, and are capable of making enormous jumps much like combat forms. A possible flaw in its AI is that it has been observed to rarely attack players, and normally prefers to mutate itself into one of the two other Pure Forms. It appears to attack more often on the levels Cortana and Halo. Even if they do attack, they are not as strong as the Ranged or Tank forms, making it the easiest of the three to dispatch.
Stalker Forms are very weak compared to the Ranged or Tank Forms, the Stalker's two choices of mutation when in combat. However, they are extremely fast and agile making it hard to hit them with slow weapons, so rapid-firing weapons are advised unless at close range. When at an appropriate distance they will mutate into the Ranged or Tank Forms which are harder to kill but are not as agile or fast as the Stalker Form.
Stalker Forms blend very well in a Flood-like environment so players should always scan the walls as they may hide themselves well enough the player may not notice them at all. Stalker Forms can attack the player, although it is quite rare and their chances of attacking the player will increase the higher the difficulty goes. However, this melee attack is relatively weak even on Legendary where a third or fourth of the player's shields are removed per hit.
- Stalker forms can be killed while mutating. As a result, the corpse left behind usually takes the appearance of the Stalker and the Ranged or Tank forms merged. Occasionally, they will have a very extended appearance with strange physics properties in-game.
- Oddly, Stalker forms, on easy will NOT attack, at extremely rare times will attack, dealing little damage.
- While annoying, agile, and creepy, the Stalker Form rarely attacks you in the levels it appears. When it does, it does not cause as much damage as it does not possess the skill set (strength, weapon compatibility, shooting needles) that the other Flood forms have as its main advantage is its speed and mobility. Stalker Forms are more likely to mutate into one of the other Pure Forms than to attack in open combat against a player.
- The Stalker Form is notable for being extremely cowardly when approached, as it will attack a player if they are unaware of the Stalker's proximity.
- The rear part of the Stalker Form seems to be parts of a Human torso.
- They cannot use any weapons apart from their pincers.
- Stalkers tend to lunge at their foes on higher difficulties, although this does not increase the damage received compared to a normal attack.
- On the Halo 3 level Halo, there seems to be an endless supply of Stalker Forms traveling up the sloped shaft in the center until the player reaches the top of the spire.
- When they have nothing to attack, they will normally scurry around waving their pincers.
- Stalker Forms have a habit of destroying equipment, namely the Deployable Cover. This can be quite annoying on higher difficulties where the player must deploy equipment to survive, especially on the level Cortana.
- On The Covenant, the allied Flood Tank Forms will sometimes mutate into Stalkers in order to attack Jump Pack Brutes. When a Brute is directly overhead, the Stalker will jump a great height in order to attack it. Though it is rarely successful, upon success, the Brute's jump pack will immediately deactivate, sending the Brute and the Stalker crashing to the ground, or down the bottomless pit.
- This is the only Flood form that can give the "assassination" medal; all other Flood forms do not give this medal, even if killed from behind.