The opalines are a small group of peculiar protists, found as endosymbionts in the gut of frogs and toads. Each cell has two or more nuclei (hundreds in some species), and is covered in short flagella, arranged in rows. As such, they somewhat resemble ciliates, but opalines have only one sort of nucleus, lack mouths, and divide longitudinally (between flagellar rows). Opalina and Protoopalina are the best known genera.

Opalines were at one point considered possible ancestors of the ciliates, but the similarity between the two appears to be convergent evolution. They are now considered close relatives of the proteromonad flagellates, and the two are included among the heterokonts, though opalines may lack the tripartite hairs that are characteristic of that group.