During the winter of 1921–22 I visited the slaughterhouse at Aberystwyth at least once each week for the purpose of studying the helminthic parasites of sheep and pigs in the neighbourhood. An average of 50 sheep and 8 pigs were examined weekly from November 26th, 1921, to April 28th, 1922. Metastrongylus elongatus and M. brevivaginatus were present in the bronchioles of 50 per cent. of the pigs examined, generally both species being present in the same animal, and I took the opportunity of working out the anatomy of these two forms in detail, and indicating the differences between them; but before being able to publish my results Gedoelst covered the ground in his “Le Genre Metastrongylus Molin, 1861,” published in 1923.