Daphnia is often coloured pink or red by haemoglobin in solution in the blood. This applies to pond-living but not to lacustrine species. A Daphnia species may vary from red to colourless in different ponds or in the same pond at different times. In the laboratory individual Daphnia can be seen to lose or gain haemoglobin in the course of a few days. Daphnia loses haemoglobin in well-aerated water and gains haemoglobin in water containing little dissolved oxygen. Abundance of parthenogenetic young may be taken as a criterion of good nutrition; judged by this standard, good nutrition alone does not result in abundance of haemoglobin. Nor is chlorophyll in food a cause of haemoglobin production. There is a haemochromogen in solution in the intestinal fluid of Daphnia. Like helicorubin in the snail it shows reversible oxidation. In quantity it is proportional to the haemoglobin of the blood, which suggests that it may be an excretory product of haemoglobin. Yet it occurs in a lake plankton species lacking the blood pigment. Occasionally one of the two excretory shell glands of Daphnia contains concentrated haemoglobin. This pathological condition may be an indication that haemoglobin is normally excreted as such by the shell glands. The presence of haemoglobin in the blood of Daphnia suggests a respiratory function. The increase in quantity of the haemoglobin in response to oxygen deficiency, just as in man, supports this thesis. Nevertheless, there appears to be no such function. Animals whose haemoglobin is functionally inactivated with carbon monoxide are as vigorous and survive as well as untreated animals, at all concentrations of air dissolved in the water. Haemoglobin is present in the parthenogenetic eggs of Daphnia as well as in the blood. Respiratory conditions in the brood pouch of parthenogenetic females are not good. This suggests an importance of haemoglobin in parthenogenetic development. Experiments in which the haemoglobin was functionally inactivated by carbon monoxide showed that the respiratory pigment of the egg does have a favourable influence on late stages of the parthenogenetic embryo. Fertilized eggs, in ephippia, contain no haemoglobin. Nevertheless, they develop as well in water deficient in oxygen as in aerated water.