One-celled organisms, also known as unicellular organisms, are found in two kingdoms:

* Monera (Prokaryotes): This kingdom includes bacteria and archaea, which are both single-celled organisms lacking a nucleus and other membrane-bound organelles.

* Protista: This kingdom contains a diverse range of single-celled eukaryotes, meaning they have a nucleus and other organelles within their cells. Examples include algae, protozoa, and slime molds.

While some protists are multicellular, many are unicellular, making both Monera and Protista kingdoms home to one-celled organisms.