Horton Hears a Who! (1954) is the second book starring Horton the Elephant. Who one day, on the fifteenth of May in the Jungle of Nool to be precise, hears a small speck of dust talking to him. It turns out the speck of dust is actually a tiny planet, home to a city called "Who-ville", inhabited by microscopic-sized inhabitants known as Whos.

The Whos ask Horton (who, though he cannot see them, is able to hear them quite well due to his extraordinary hearing) to protect them from harm, to which Horton happily obliges, proclaiming throughout the book that "a person's a person, no matter how small". In doing so he is ridiculed and nearly murdered by the other animals, such as the Wickersham Brothers and the Sourpuss Jane Kangaroo, and the small kangaroo in her pouch, in the jungle for believing in something that they are unable to see or hear.

Horton tells the Whos that they needed to make themselves heard to the other animals, lest they end up as part of "beezlenut stew", which they finally accomplish. The Who's finally make themselves heard by ensuring that all members of their society play their part. In the end it is the smallest Who of all, Jo-Jo, who provides the last volume lift to be heard, thus reinforcing the moral of "a person's a person no matter how small".