Capillaria is a genus of nematodes in the family Capillariidae (or, according to classifications, in the family Trichinellidae).
Since the taxonomy of the Capillariidae is disputed, species are included within the single genus Capillaria or 22 different genera (Amphibiocapillaria, Aonchotheca, Baruscapillaria, Calodium, Capillaria, Capillostrongyloides, Crocodylocapillaria, Echinocoleus, Eucoleus, Freitascapillaria, Gessyella, Liniscus, Paracapillaria, Paracapillaroides, Pearsonema, Paratrichosoma, Pseudocapillaria, Piscicapillaria, Pseudocapillaroides, Pterothominx, Schulmanela, and Tenoranema).
Some species parasitic in fish, previously classified within Capillaria, are now included in Huffmanela (family Trichosomoididae).
Old literature, and sometimes modern medical literature, use Capillaria as a genus for species included in all these genera.
The term Capillariasis is generally used for diseases produced by species of Capillaria, even if the species is now placed in another genus.