In the rainbow trout, Salmo gairdneri irideus, round, disc-like erythrocytes in the embryonic circulation (Ery L) are replaced by small, elliptical, disc-like erythrocytes (Ery ImA) after hatching. In the peripheral blood of alevins, Ery ImAs grow into mature, adult erythrocytes (Ery A) of large elliptical, disc-like shape (Iuchi, '73b; Yamamoto and Iuchi, '75). Ery L and Ery A have larval and adult hemoglobins, respectively (Iuchi, '73a). The ontogenetic sequence of hemoglobin switching and erythrocyte replacement during erythropoiesis was examined by o-dianisidine histochemistry as well as immunohistochemistry using FITC-antibody probe specific to adult hemoglobins. The first phase of embryonic erythropoiesis (for Ery L) occurs in the intraembryonic "intermediate cell mass" as well as the extraembryonic blood islands on the yolk sac. This phase of erythropoiesis is transient, continuing during the 7th to 12th day after fertilization and ceasing by the 15th day (5th day before hatching). There is a mixed population of Ery L, ImA, and A in the peripheral blood of posthatching alevins. Ery ImA and A showed immunofluorescence with FITC anti-Hb A antibody but Ery L did not. Erythroid cells, stainable with FITC anti-Hb A antibody, were observed in the kidney and the spleen 1 day before hatching and thereafter, but not in the liver throughout all stages examined. Therefore, we conclude that new erythropoiesis (erythropoiesis for Ery ImA and Ery A) began in the kidney and the spleen 1 day before hatching. These findings indicate that hemoglobin switching during the ontogeny of rainbow trout is based on erythrocyte replacement, correlated with a shift in the site of erythropoiesis from one organ to another.