Pelomyxa is a genus of giant flagellar amoebae, usually 500–800 μm but occasionally up to 5 mm in length, found in anaerobic or microaerobic bottom sediments of stagnant freshwater ponds or slow-moving streams.

A moving cell is cylindrical in shape, with a single hemispherical pseudopod at the front and a semipermanent projection called a uroid at the back, which is covered in tiny non-motile flagella.

Pelomyxa lack mitochondria, as well as several other organelles usually found in eukaryote cells (notably, peroxisomes and dictyosomes).

While the function of these was unclear, Whatley argued that they might provide a useful evolutionary example, indicating the "ways in which a bacterial mitochondrial transformation might have been attained.

"[15] In 1982, Lynn Margulis created the subclass Caryoblastea (or Pelobiontidae) for "anaerobic ameobas that lack undulipodia," and assigned Pelomyxa to it as the only member of the group.

[18] Consequently, Pelomyxa and the other Archamoeba were reassigned to the phylum Amoebozoa, under the subphylum Conosa (shared with the Mycetozoan slime moulds).