My paper proposes to highlight notions of the nature and role of aquatic animals in Claudius Aelian's On the Characteristics of Animals, by focusing on the role medical knowledge plays in their presentation. In the first instance, I will show that medical vocabulary is used in contexts where Aelian offers narratives which stress the harmful nature of aquatic animals (e.g. cases where their bite or touch is harmful to humans). A characteristic example is the aristotelian testimony (fr. 368) in chapter 4.57 that the bite of a water snake causes an intolerable bad smell (παραχρῆμα ὀσμὴν βαρυτάτην ἀπεργάζεσθαι), loss of memory and sight (λήθην τε καταχεῖσθαι [...]ἀχλὺν κατὰ τῶν ὀμμάτων πολλήν), madness and fear (λύτταν ἐπιγίνεσθαι καὶ τρόμον εὖ μάλα ἰσχυρόν) and finally death in the course of three days (ἀπόλλυσθαι διὰ τρίτης αὐτόν) to the bitten person. Secondly, I will discuss narratives about remedies for different human diseases through the use of aquatic animals as medicines, which show their benefit for humans. For example, Aelian notes in chapter 14.15 that the human disease of opthalmia is cured by using the eyes of the parrot-wrasse (ὀφθαλμὸς δὲ ἄρα ὁ τούτου (τοῦ σκάρου) ὁπότερος οὖν ἐξαιρεθεὶς καὶ περίαπτον γενόμενος ἀπαλλάττει ξηρᾶς ἄνθρωπον ὀφθαλμίας). These cases will be compared with pharmacological knowledge found both in imperial medical works, such as Galen's treatises, and in Pliny the Elder's HN (books 31-32), where aquatic animals are presented in similar terms. As it will be suggested, Aelian's narratives about the medicinal uses of aquatic animals adopt an anthropocentric scope, which can also be found in other cases in his work – for example, when animals are presented in connection with aspects of human life (e.g. religion, hunting and trade).