A number of filarial worms were collected from the mesentery of a giant anteater, Myrmecophaga tridactyla, which died in the London Zoo four months after its arrival.Genus Derdiophoronema Romanovitch, 1916 Deraiiophoronema freitaslenti n.sp. One complete male, two complete females and a number of pieces were available for study. These long worms are blunt anteriorly, but taper down gradually at their posterior ends in both sexes. The cuticle is smooth and devoid of any striations. The head-end has two rudimentary lateral lips, each with five papillae. There is a small buccal cavity. The mouth opening is oval, with the longer diameter dorso-ventral.Female. The female is 62-0-68-2 mm. long and 0-33-0-36 mm. in maximum diameter. The oesophagus is 9-6-10-7 mm. long. The short anterior muscular part is 0-38-0-50 mm. The nerve ring is 0-21-0-25 mm. from the anterior end. The tail is 0-25-0-34 mm. long, and ends in a small single terminal spike. Thenon-protrusible vulva opens 2-1-2-4 mm. from the anterior end of the worm. The vagina and uterus may be directed immediately posteriad, or more usually, in the older specimens, it is directed as far anteriorly as the nerve ring before turning backwards when it then branches into two uteri. The anterior end of the vagina is more muscular and bulbous than the rest. The microfilaria is sheathless.Male. The male is 37-5 mm. long and 0-27 mm. in maximum width. The oesophagus is 9-8 mm. long. The short anterior muscular portion is 0-42 mm. The nerve ring is 0-21 mm. from the anterior end. The cloacaopens 0-15 mm. from the posterior tip. The tail is provided with alae which are supported by four pairs of pre-cloacal papillae, six pairs of post-cloacal papillae, a single median pre-cloacal papilla, and a pair of papillae behind the cloaca, making a total of twenty-three papillae. The spicules are dissimilar and very unequal. The short, broad right spicule is distinctly curved and has a little notch before its tip. This spicule is 0-160 mm. long and 0-021 mm. wide. The long left spicule is 0-670 mm. It is pitted anteriorly and has a twist half-way down its length, and branches into two parts. The main part