The infective larva of Ancylostoma braziliense Gomes de Faria, 1910, was compared with that of A. ceylanicum Looss, 1911, as seen with the light microscope and scanning electron microscope. The following differences were found: (1) With or without the sheath, A. ceylanicum is significantly longer than A. braziliense. (2) The distances from anus to caudal tip of the body and from there to the tip of the sheath are both significantly greater in A. ceylanicum than in A. braziliense. (3) The transverse striations on the sheath are more pronounced in A. ceylanicum than in A. braziliense. These findings further demonstrate that A. braziliense and A. ceylanicum are distinct species, and show that their infective-stage larvae are easily distinguishable. Considerations of the synonymy of Ancylostoma ceylanicum Looss, 1911, and Ancylostoma braziliense Gomes de Faria, 1910, have usually been based on the morphology of the adult worms (Yoshida, 1971). Few observations have been made on the morphology of these two ancylostomes in the infective larval stage, although Kobayashi (1928) in Taiwan made measurements on A. ceylanicum infective larvae and compared them with those of A. duodenale, A. caninum, and Necator americanus. The present investigation was carried out in an attempt to clarify the larval morphology of A. braziliense and A. ceylanicum, and to determine the features by which the two species can be differentiated in the infective stage. MATERIALS AND METHODS The strains of A. braziliense and A. ceylanicum used in the present research were the same as those studied earlier (Yoshida, 1971). The infective larvae were obtained by cultivation (HaradaMori method) of eggs in feces from dogs having pure infections. They were examined 2 to 3 weeks after initiation of the cultures, both with the light microscope and with the scanning electron microscope. Prior to examination with the light microscope the larvae in a drop of water on a slide were killed by heat, then covered with a coverglass. They were examined within 15 min after heat fixation. For study with the scanning electron microscope, the larvae were fixed in 2% glutaraldehyde for 1 hr, then dehydrated with acetone. The dry materials were coated first with carbon then with gold in the vacuum chamber. The microscope used was a JSM-2 model of Japan Electron Optics Company. Received for publication 18 March 1971. RESULTS