IT is known that the amount of haemoglobin dissolved in the blood of Daphnia varies inversely with the oxygen content of the water in which the animals live. The changes in concentration of the haemoglobin have been studied by a quantitative method which allows measurements to be made on a single Daphnia : the colour of the animal‘s blood is compared under the microscope with known dilutions of human blood1. In the course of further work involving the use of this method it was noticed that parthenogenetic females with eggs in the brood pouch have less haemoglobin in their blood than those with embryos in the pouch. The difference is greatest in populations producing numerous young.