The Labyrinthulomycetes are a group of protists that produce a network of filaments or tubes, which serve as tracks for the cells to glide along and absorb nutrients for them. There are two main groups, the labyrinthulids and thraustochytrids. They are mostly marine, commonly found as parasites on algae and seagrass or as decomposers on dead plant material. They also include some parasites of marine invertebrates.

Although they are outside the cells, the filaments are surrounded by a membrane. They are formed and connected with the cytoplasm by a unique organelle called a sagenogen or bothrosome. The cells are uninucleate and typically ovoid, and move back and forth along the amorphous network at speeds varying from 5-150 μm per minute. Among the labyrinthulids the cells are enclosed within the tubes, and among the thraustochytrids they are attached to their sides.

The Labyrinthulomycetes were originally considered unusual slime moulds, although they are not very similar to the other sorts. The structure of their zoospores and genetic studies show them to be a primitive group of heterokonts, but their classification and treatment remains somewhat unsettled.