While there are other kingdoms that contain some single-celled organisms (like bacteria in the Kingdom Monera or some fungi), the vast majority of single-celled organisms are classified under the Protista kingdom.

Here's why:

* Protists are a diverse group of eukaryotic organisms (meaning they have a nucleus) that are mostly unicellular. They include organisms like amoeba, paramecium, algae, and slime molds.

* Monera (also known as bacteria) are prokaryotic, meaning they lack a nucleus.

* Fungi are mostly multicellular, though some yeasts are single-celled.

So, although there are exceptions, Protista is the kingdom that primarily encompasses the world of single-celled organisms.