Eelworm, a common name for a free-living (that is, nonparasitic) or plant-parasitic roundworm. Eelworms are tiny, often barely visible to the unaided eye, with threadlike bodies. Free-living eelworms can be found in water and soil; a common free-living species is the vinegar worm, or vinegar eel, which is often found in unsterilized vinegar. Eelworms that are parasitic on plants attack both roots and stems. Those that infest grasses and grain crops cause galls, or swelling in the stem.

Eelworms belong to the phylum Nematoda. The vinegar worm is Anguillula aceti.