The bio-suit (also called a mech prawn), as the name says, is a mech used by the Prawns to defend, attack or help in construction and repairs.
The appearance of the bio-suit in District 9 is greatly changed from that of the short film. The suit now appears to be more of a mechanized piece of equipment, and is much larger than the bio-suits of Alive in Joburg. Instead of simply wearing the suit as one would with the bio-suits of the short film, the new suits have a central chamber in the thoracic cavity of the machine where the operator resides. Inside the machine, the operator has complete control over a wide variety of tools and weapons on the suit which are all controlled via the highly advanced heads-up display. The mechanized suit could presumably be used for tasks such as ship and technology repairs, but the suit also has a wide arsenal of highly dangerous offensive and defensive weapons. The suit has been shown to posses a larger version of the ARC gun, a machine gun, rocket launchers and a metal spike that drives into the enemy and sends a charge through them that eventually causes them to explode. The suit can also activate certain weapons on its own when a prawn (or one with prawn DNA) is attacked. Once it eliminates all threats to the threatened prawn, the cockpit opens allowing the prawn to climb in and use the mech for self-defense.
The bio-suit has an in-built AI auto-pilot, but the motion of the suit in this state is more robotic. The bio-suit is totally interactive with the driver when used, for example when Wikus is damaged using the suit, a fluid sprays and the suit walks lame like a damaged creature. When the bio-suit is activated in the film, it quickly dealt with Wikus's human attackers, identifying only him as having prawn DNA. It then presents itself for his use, maybe because the reactions of the AI are not as effective as those of the non-humans.
The weapons on the suit (or at least the ones that have been seen being operated) appear to be similar to standard alien weaponry, but they are integrated into the suit. A variant of the ARC Gun, a chaingun-like weapon, and missiles are the weapons that have been seen on the bio-suit. A strange defensive mechanism is also present on the suit which appears to be a telekinetic field. This magnetic field can completely control the paths of bullets and other objects, making the suit nearly untouchable by firearms. Once the mechanism has gained control over an object or objects, it possesses the ability to launch the objects back at targets with great accuracy and velocity. Wikus uses this tool to launch a pig at attacking MNU forces and launch the bullets of the Nigerians back to them.
The suit appears to need a direct connection to the mind of the operator as when Wikus starts it up, two rods are drilled into his head, removing themselves when the exosuit breaks down.
The Nigerian gang end up getting one of these from the non-humans as part of an attempt to trade for food. During the end of the film, Little CJ activates it as the Nigerians are about to kill and eat Wikus. The suit automaticaly kills all of the Nigerians and opens itself to Wikus. Finding himself surrounded, Wikus uses the suit to escape and rescues Christopher Johnson. Wikus covers Christopher until he's close to the drop ship, but is forced to have Christopher go on without him as the suit has by that point taken a lot of damage and can't go very fast anymore. Wikus uses the suit to battle MNU's forces, killing all but their leader Venter, but after getting shot multiple times, including with a high-powered sniper rifle, had its left hand blown off by a rocket and rammed twice with trucks, the suit breaks down and ejects Wikus.