In some investigations on skin penetration by the filariform larvæ of Strongyloides (Goodey, 1925), the writer made use of larvæ cultivated from the droppings of a Guinea Baboon, Papio papio, which had died at the Gardens of the Zoological Society of London. Scrapings of the lining of the small intestine of this animal were made so as to secure the parasitic worms of which 25 specimens were obtained. A study of these was undertaken because von Linstow's (1905) original description of the species is so slender and his figures illustrating it are so crude that it was considered that the parasite would repay further investigation. Moreover one or two of his statements as, for example, that the cuticle of the parasite females is unstriated, required critical examination. Again in a recent paper Chandler (1925) has thrown doubt on the specific identity of S. fülleborni, suggesting that it is probably a hostal variety of S. papillosus. For these reasons the present paper has been prepared in that it gives a fuller account of S. fülleborni both in the parasitic and in the free-living generation than has hitherto been available. The writer considers that a detailed and well illustrated account of the morphology of S. papillosus, particularly of the free-living, sexual generation, is much needed at the present time.