The Humming Fish are a species of amphibious fish that inhabit the lakes in the Truffula Tree forest.
The Humming fish are first seen when the Once-ler came to the Truffula Tree forest and began to sing to greet him. After the other animals and them briefly sang they tried to attack the Once-ler but they calm down when they tasted his marshmallows. They then watch in horror as he cuts down the first Truffula tree and mourn the loss of the tree with The Lorax.
Later that night after they helped the Lorax put the Once-Ler's bed in the water they (along with The Lorax and other forest creatures) save the Once-Ler from going over the waterfalls. Later they watch in horror and sadness as the Once-Ler (under the influence of his greedy and lazy relatives) cuts down all the trees and pollutes the lakes and rivers in which they swim in. After the last Truffula Tree is chopped down the Lorax send them and the other forest creatures off to find another place to live.