Anguilliformity is a morphological pattern in fishes, named for and typified by the eels. Anguilliform fish have a long, slender body, and travel by anguilliform motion. The caudal fin is often emphasized, with the other fins reduced, absent, or fused with the caudal fin. ^[1] Anguilliformity has evolved independently in many groups, including among others:

• Anguilliformes, the eels
• Synbranchiformes, the swamp eels
• Clariidae, the airbreathing catfishes
• Dipnoi, the lungfishes
• Cobitidae, the loaches
• Gymnotidae, the knifefishes, including the electric eel Electrophorus electricus