An investigation was made in both the parasitic lamprey L. fluviatilis and in its non-parasitic derivative L. planeri of the rate at which the fat column replaced the typhlosole and nephric fold as the principal site of hemopoiesis. In the typhlosole, blood cell formation started to decline prior to the onset of external metamorphosis and had ceased within 4 wk of the commencement of transformation. In the nephric fold hemopoiesis continued for several weeks in the region where the larval opisthonephros persisted but was never observed in the newly developing adult kidney. Soon after the onset of external metamorphosis the fat column became hemopoietic and later became the main site of blood cell formation. The rate at which the hemopoietic function was transferred from the nephric fold and typhlosole to the fat column was greater in L. fluviatilis than in L. planeri. Since a similar more rapid change in L. fluviatilis was also found in the switch from larval to adult Hb, the former type of Hb may be produced only in erythrocytes originating in the nephric fold and typhlosole, whereas the latter may be restricted to cells developed in the fat column. The functional significance of the alteration in hemopoietic sites was related to changes at metamorphosis in the 3 regions where blood cell formation occurred.