Macrostomidae[1] is a family of small basal free-living flatworms, and are found in marine, brackish, and freshwater environments.

[2] Species of the family Macrostomidae are small (~0.5 to 5 mm in length) and generally highly transparent microturbellarians.

They are distinguished from related animals by the possession of a simple pharynx and intestine, a single pair of lateral nerve cords, and by the absence of a statocyst.

Moreover, they occur in marine, brackish, and freshwater habitats, and many forms are particularly abundant in the intertidal meiobenthos, where they can be exposed to highly variable environmental conditions.

They can be found in all major bodies of water worldwide, and in the freshwater of all continents, except Antarctica.