The basilisk is described as a crested snake or as a cock with a snake's tail. It is called the king (regulus) of the serpents because its Greek name basiliscus means "little king"; its odor is said to kill snakes. Fire coming from the basilisk's mouth kills birds, and its glance will kill a man. It can kill by hissing, which is why it is also called the sibilus. Like the scorpion, the basilisk likes dry places, and its bite causes the victim to become hydrophobic. The poison of the basilisk is so strong that if one is stabbed with a spear the poison will flow up the spear to infect the person holding it. The touch and breath of the basilisk kills bushes, scorches grass and breaks stones. Other animals will not eat or even touch any animal that was killed by a basilisk.

The only animal that can directly kill a basilisk is the weasel. The weasel enters the cave where the basilisk lives, causing the basilisk to flee, but the weasel catches and kills it; Sometimes the weasel may itself be killed in the fight. Basilisks are afraid of the cock and shudder when they see one; the crowing of a cock causes convulsions in the basilisk, from which it sometimes dies.

The basilisk is hatched by a toad from a cock's egg, a rare occurrence. When the egg hatches, a male chick emerges; after seven days it grows the tail of a serpent.

Some manuscripts have separate entries and/or illustrations for the basilisk and the regulus, possibly because the basilisk account in Isidore has three sections, one each for the basilisk, the "kinglet" (reguli), and the sibilus. Where the regulus is treated separately, the bite of the basilisk causing hydrophobia is generally ascribed to the regulus.

Basilisk illustrations vary greatly. Most show a bird-snake hybrid, with the beast being a bird (often a cock) with a snake tail. In some images the basilisk is all snake with no bird parts. The basilisk as king of the serpents appears in some manuscripts, with the basilisk wearing a crown. The weasel attacking the basilisk is commonly included in the image.

By following a complex alchemical formula (see Sources) involving the ash of burned basilisks, copper or silver can be made to to appear to be pure gold.