Bigyra (from Latin bi- 'twice' and gyrus 'circle')[1] is a phylum of microscopic eukaryotes that are found at the base of the Stramenopiles clade.

It includes three well-known heterotrophic groups Bicosoecida, Opalinata and Labyrinthulomycetes, as well as several small clades initially discovered through environmental DNA samples: Nanomonadea, Placididea, Opalomonadea and Eogyrea.

[4] It contains three well-known important groups with widely different ecological functions and morphologies: labyrinthulomycetes, opalines and bicosoecids.

Among them, the opalinids are highly unusual protists: their large cells have numerous flagella and from two to hundreds of nuclei.

They play a crucial role in the microbial food web by composing the link between bacteria and higher trophic levels.

[8] The Stramenopiles are a supergroup of eukaryotic organisms (protists) characterized by the presence of an anterior flagellum with tripartite hairs, called mastigonemes.

Opalozoa is further subdivided into two groups: Placidozoa, which contains the opalines and three clades discovered through the detection of environmental DNA (Nanomonadea, Opalomonadea and Placididea), and the bicosoecid flagellates.

The positions of the two bigyran clades (Opalozoa and Sagenista) are not consistent between the published studies, because they diverged from each other very early after the separation from the ancestor of all stramenopiles.

Bigyra was, at the time, postulated as a monophyletic group (or clade), evolved from a paraphyletic grade of ochrophyte classes.

They revealed Pseudofungi and Bigyromonadea were more closely related to a monophyletic Ochrophyta than they were to Opalinata, meaning that the synapomorphy of a double helix could have been present in the common ancestor of all heterokonts.