Two members of a collection of the South American rodent Viscacia viscacia received by the Zoological Society of London, died early this year, one of them about 10 days after arrival at the Gardens. At the post-mortem examination both were found to have the alimentary canal heavily parasitized with Trichostrongylid nematodes. In the stomach there occurred Graphidioides rudicaudatus and Trichostrongylus retortæformis, whilst in the small intestine there were a few specimens of Trichostrongylus retortæformis and large numbers of the worm which forms the subject of the present paper. To naked eye examination of the intestinal wall the worms appeared as small bright red spots about the size of a pin's head and it could be seen that each was spirally coiled.