In 5- to 7-da old opossum pouch young, hemoglobins were observed. The "primitive" species I, II, and IV were shown to be electrophoretically distinct from the adult hemoglobin III. Hemoglobin I was detectable until the 7th postnatal da. Hemoglobins I, II, and IV were detected only during the period when yolk sac-derived primitive erythroblasts were seen in circulation, ei, until the 13th postnatal da, but in decreased amounts from the 6th to 13th da. The electrophoretic pattern of hemoglobin of pouch young after the 13th postnatal da was indistinguishable from that of the adult opossum.