The site of maturation of the erythroid cells in premetamorphic, metamorphic, and adult stages of R. catesbeiana has been investigated by comparing the percentages of immature, hemoglobin-containing erythroid cells in circulating blood, liver, spleen, kidney, and bone marrow. In both premetamorphic and thyroxine-treated tadpoles, the liver was found to be the main site of maturation of the red cells. The corresponding site in the frog is the bone marrow.