Two young pigs used by the writer in an attempted experimental infection with Necator americanus proved to be carrying a natural infection of Œsophagostomum dentatum. At the post-mortem examination, one of the animals had 35 females, 22 males, all mature worms, in the cæcum and large intestine, whilst two pairs were in copula. From the cæcal contents of the other pig, one immature female worm was obtained. No nodules were found in the walls of the cæcum or of the large intestine. Droppings containing the eggs of the parasite had been collected for a short time prior to the killing of the pigs, and were used for the cultivation of the larvæ.

In the following report 38 species of helminths from vertebrate hosts are considered, of which three are new. All were collected by Dr. C. Bonné of Surinam, and were presented through Prof. Leiper, F.R.S., to the London School of Tropical Medicine. The writer wishes to express his thanks to Prof. Leiper for the privilege of examining this material.

This parasite was described in 1910 by Railliet and Henry from material sent to them by Gaiger from India. Gaiger, in 1911, published a translation of the original description, together with some figures. In 1916, Gedoelst reported its occurrence in the Belgian Congo, but without adding to the description of the worm.

As there is no complete or adequate description of this parasite of dogs and foxes, and as it is proving to be of considerable importance in the newly established fox-breeding industry, it was thought advisable to prepare an account of its morphology. The material from which this description was made was obtained from a British fox which died at the Zoological Gardens, London, in June, 1923.