Macrostomum[1] is a genus of flatworm with a worldwide distribution,[2][3][4] with over a hundred species described to date.

[2] These hermaphroditic, free-living flatworms are usually small in size, with large species reaching up to 5 mm in body length (e.g. Macrostomum tuba).

The term "Macrostomum", meaning "big-mouthed", derives from the Greek μάκρος makros, "large", and στόμα, stoma, mouth.

Many of these small worms feed on unicellular algae such as diatoms, others on zooplankton or smaller benthic invertebrates.

[3] This genus includes Macrostomum lignano,[5] a new model organism for studies on different areas of biology, including development,[6] bioadhesion,[7] regeneration,[8] stem cell biology,[9] ageing,[10] toxicology,[11] genomics,[12] and evolution.