- any of various small, red annelid worms, as the sludgeworm
- the wormlike red larva of various midges, esp. of a genus (Chironomus) that lays its eggs in water
- Any of various marine polychaete worms of the genus Glycera, having bright red bodies and often used for bait.
- Any of various freshwater larvae of certain midges that are red as a result of the high hemoglobin content of their blood.
- Any of various parasitic worms of the genus Strongylus that infect the intestinal systems of horses. Also called redworm .
- Alternative spelling of blood worm.