Professor Magalhăes describes two sexually mature filarial hæmatozoa, male and female, which were found lying in the left ventricle of the heart of a child in Rio de Janeiro. No information was received as to the nature of the disease of which the child died, nor had any examination of the blood been made during life. The parasites were cylindrical, capillary, opalescent, white, uniform in thickness except, where the body tapered towards the tail and at the club-shaped oral end. The mouth was simple, circular, unarmed; the cuticle marked with fine transverse striations. The female worm measured 155 mm. in length by 0·7 mm. in diameter, the male 83 mm. in length by 0·4 mm. in diameter. The tail of the latter was provided with four pairs of preanal and four pairs of postanal papillæ, and two spicules. Manifestly this parasite is specifically distinct from F. bancrofti and the other blood worms described above. Nothing is known of its life-history, nor of the associated pathology.