Another unique feature of the park is associated with its hot springs, but is not a geological phenomenon. Instead, this feature is biological. A kingdom of microorganisms, similar in some ways to bacteria, but very different in some ways, is found in abundance in these environments which would kill most living things. The name of the kingdom is Archaea. The kingdom is generally made up of extremophiles, such as those organisms that can live in extremely salty or acidic environments. The kingdom also includes organisms called thermophiles that can withstand temperatures approaching, and even exceeding, the boiling point of water. These types of organisms are found all over Yellowstone, and are the cause of the brilliant colors found in the picture above.

The park is also home to many of the continent's large mammal species, such as bears, bison, cougars, elk, and wolves.