1. 1. In Geotria australis, the haematocrit, red blood cell number and haemoglobin concentration of adults (46.2%, 1.231 × 10 6 cells/mm 3 and 11.8 g/100ml) are more typical of comparable stages in holarctic lampreys than are those of their ammocoetes (41.5%, 1.809 × 10 6 cells/mm 3 and 11.1g/100ml). 2. 2. Mean corpuscular volume, mean corpuscular haemoglobin and mean corpuscular haemoglobin concentrations in larvae and adults were respectively 228.4 and 369.5 μm 3, 63.7 and 94.1 pg and 27.9 and 25.5%. 3. 3. Haematocrit and red blood cell number declined and then rose during metamorphosis. 4. 4. The pattern of change during metamorphosis in haemopoietic sites, haemoglobin electropherograms and the proportion of mature erythrocytes, indicate that erythrocytes containing larval and adult haemoglobins always originate in different structures.