Hemoglobin could not be detected in the infective larvae of Obeliscoides cuniculi. It was first detected in the 4th larval stage approximately 6 days after host infection and thereafter its concentration rapidly increased. Hemoglobin was present in the perienteric fluid and in the body wall of both female and male adult worms. Hematin, detectable histochemically, was present as inclusion bodies in the intestinal cell nuclei of all stages that contained hemoglobin. Iron-containing pigment granules appeared within the cytoplasm of intestinal cells of 4th stage larvae at about the time hemoglobin was first detected in the larvae. These pigment accumulations continued to increase throughout development. The above findings are discussed in relation to the possible site of synthesis of worm hemoglobin. Spectra for several common derivatives obtained with partially purified perienteric fluid hemoglobin are presented. The oxyhemoglobin spectrum differed considerably from that of mammalian hemoglobin whereas the spectra of the other derivatives studied did not. Hemoglobins occur in many endoparasitic nematodes (Gratzer and Allison, 1960, see Lee and Smith, 1965 for review). Very few of these hemoglobins have been adequately investigated with respect to their function in vivo. All nematode hemoglobins studied so far have high affinities for oxygen (P50 of 1 to 2 mm) (Rogers, 1949a; Lee and Smith, 1965; Okazaki and Wittenberg, 1965). However, Wharton (1941), Davenport (1949a, b), Rogers (1949b), and van Grembergen (1954) observed that the body wall hemoglobins of the nematodes they studied could be deoxygenated by exposing live worms to low oxygen tensions. Davenport (1949a, b) found that the perienteric fluid hemoglobin of Ascaris lumbricoides and Strongylus sp., as opposed to the body wall hemoglobins, did not deoxygenate in an anaerobic atmosphere even when the worms themselves were moribund. In fact, Okazaki and Wittenberg (1965) had to first convert Ascaris perienteric fluid hemoglobin to the carboxy form and then dissociate the carbon monoxide hemoglobin with light in order to obtain deoxygenated hemoglobin. Similar observations led Smith and Lee (1963) to consider the perienteric fluid hemoglobin of Received for publication 13 May 1968. * This work was supported in part by research grant A3895 from the National Research Council of Canada and by the Ontario Department of Agriculture and Food. A. lumbricoides to be a source of hematin for the ggs and a general metabolic pool of hematin for other tissues. Thus, although many workers agree that the body wall hemoglobins of parasitic nematodes may be effective oxygen carriers at the low tensions prevailing in the host intestine, it is not possible with the data available at present to describe precisely the function or functions of their perienteric fluid hemoglobins. The present work was therefore initiated to study further the origin, distribution, metabolism, and certain properties of the hemoglobins of parasitic nematodes with a view to better understand the physiological importance of this pigment to the parasites in vivo. The rabbit stomach worm, Obeliscoides cuniculi, was found to contain a high concentration of hemoglobin and as this nematode was obtainable in large enough numbers from experimentally infected laboratory rabbits it was used extensively in most of the initial work. In the present paper the distribution of hemoglobin and other hematin compounds within the body of 0. cuniculi, their occurrence and accumulation during larval development and the isolation and some of the properties of the perienteric fluid hemoglobins are described. The ontogeny of the multiple hemoglobins found in the adult worms, their possible origin, physicochemical properties and oxygen relationships will be the subject of further papers.