The nucleariids are a small group of amoebae with filose pseudopods, known mostly from soils and freshwater. They are distinguished from the similar vampyrellids mainly by having mitochondria with discoid cristae. Genetic studies place them near the animals and fungi.

Nucleariids are usually small, up to about 50 μm in size. Nuclearia and Vampyrellidium, which are respectively parsitic on and in algae, take the form of naked amorphous cells. The other genera, Rabdiophrys, Pinaciophora, and Pompholyxophrys, are freshwater forms with hollow siliceous scales or spines. These were formerly included among the heliozoa as the Rotosphaerida.