Pond-dwelling species of Daphnia are known to synthesize blood haemoglobin in poorly aerated natural waters. It has now been found that a lake plankton species, the blood of which is colourless in nature, also synthesizes haemoglobin in artificial conditions of oxygen deficit. Haemoglobin formation in Daphnia, resulting from a low oxygen content of the water, increases with the amount of algal food eaten, up to a certain nutritional level. Chlorophyll in the food has not been found to favour haemoglobin synthesis. In the water of ponds harbouring red Daphnia we have found no factor stimulating haemoglobin synthesis other than oxygen deficit. The haemoglobin content of the blood of Daphnia varies within each instar. It is least when eggs are laid by the parthenogenetic female in her brood pouch and greatest when the eggs have developed into late-stage embryos ready to be released. After this, during the few hours before the moult and the laying of more eggs, the haemoglobin content of the blood decreases rapidly. At the same time the ovaries gain haemoglobin. This implies that haemoglobin passes from the blood into the ovaries shortly before the eggs are laid. During the development of the eggs in the brood pouch, the blood of the female gradually reacquires its full haemoglobin content.