The asp is a serpent that avoids the enchantment of spells or music by pressing one ear against the ground and plugging the other ear with its tail. In some versions the asp guards a tree that drips balm; to get the balm men must first put the asp to sleep by playing or singing to it. Another version holds that the asp has a precious stone called a carbuncle in its head, and the enchanter must say certain words to the asp to obtain the stone.

Asps have great affection for their mates. If one serpent of the pair is killed, the other pursues the killer with great a great desire for revenge. The asp can recognize its enemy even in a crowd of people, and will overcome all obstacles to attack him. The vengeful asp is only stopped by rivers or by the rapid flight of the one it is chasing.

The asp has several varieties, each with its own name.

• Haemorrhois or emorrosis: so called because it kills by making you sweat blood; "haem" or "heem" is the iron part of blood. Its bite makes the victim grow weak, so that his veins open and his life pours out with his blood.
• Prester: an asp that moves quickly with its mouth always open and emitting vapor. When it strikes, the body swells up and the victim dies of gross distention, and the swollen body putrefies immediately.
• Hypnalis or ypnalis: kills by sending its victim to sleep. It was this snake that Cleopatra applied to herself, and was "released by death as if by sleep."
• Spectaficus: An asp whose bite causes body and bones to dissolve.
• Albertus Magnus describes a kind of asp called the swallow snake (irundo, a name also used for the swallow and the sea-swallow).

The asp represents the worldly and wealthy, who keep one ear pressed to earthly desire, and whose other ear is blocked by sin.

The usual illustration of the asp shows the enchanter reading from a scroll or playing a musical instrument, with the snake at his feet blocking its ears. As second type of illustration shows the enchanter touching or hitting the asp's head with a stick or a wand. The asp guarding the balsam tree is occasionally illustrated.

For the illustration of the asp in British Library, Harley MS 4751, folio 61r, the scroll the enchanter is reading from has writing on it. The writing says Super aspidem et basiliscum amb.; the full quote is Super aspidem et basiliscum ambulabis, et conculcabis leonem et draconem (Upon the asp and basilisk shalt thou tread, and thou shalt trample upon the lion and dragon). This is from the Christian bible, Psalm 91, verse 13. This verse was used in a monastic chant, which suggests the man is singing the words to enchant the asp. In Kongelige Bibliotek, GKS 1633 4° (folio 52r) the enchanter also has a scroll with two words on it (canta serpens?). In Museum Meermanno, MMW, 10 B 25 (folio 41r) there is a scroll with illegible writing on it. In other manuscripts the enchanter has a book or other text with no words visible.

Some sources say the asp has a stone in its forehead that is valued by people. The stone is called the carbuncle, though what it is used for is not explained.