Their appearance is similar to a floating spheroid with a filamentous trunk (which appears to be made of a web of tendrils) hanging downwards. They also have what appears to be a pair of glowing eyes and a pair of flat nostrils. The Melkots are powerful telepaths, able to create elaborate illusions which affect even other telepathic species (such as Vulcans). They consider other species as "disease", seeing them as dangerous and belligerent. Alien ships which enter Melkotian territory are given a single warning to withdrawn.
When the USS-Enterprise ignored a Melkotian warning in the year 2268 and transported a landing party consisting of Captain Kirk, Spock, Dr. McCoy, Scotty and Chekov to an unnamed Melkotian planet; the aliens punished them by creating an illusory representation of the Earth city of Tombstone, Arizona, in the year of 1881, when a gunfight occurred between the Earps and the Clantons. The landing party were given the role of the Clantons, who historically were killed in the combat, and believed that their execution was to be played for real after Ensign Chekov was shot and apparently died. However Spock theorized that they only saw him die because they believed the objects were real and the bullets were solid. By performing mind melt with the remaining party, Spock allowed them to disbelieve the illusion and survive the gunshots, thus giving victory to the Clantons. Kirk's display of mercy towards the Earps' leader, even though he wasn't real, ultimately convinced the Melkots that the Federation had peaceful intentions. As the illusion ended and Chekov was returned to normal, the Melkot representative invited the Enterprise to his homeworld, where a diplomatic delegation was willing to make contact. It is unknown if the Melkots joined the Federation afterwards, but it seems likely that they at least became allies.