1. Daphnia magna can be grown for at least 200 generations, axenically or in crude cultures, in a defined mineral medium, enriched with vitamin B12 and thiamine and 1 ml/100 of a dilute organic medium, when fed with Chlamydomonas reinhardi and Scenedesmus obliquus.2. The organic enrichment is essential for maintaining continued fertility of D. magna.3. The organic enrichment can be replaced by the addition of pantothenic acid to vitamin B12 and thiamine without lowering the fertility of D. magna.4. The organic enrichment (or the vitamin mixture) does not act directly on D. magna but via the algae by changing their nutritional value for Daphina.5. Addition of vitamins to the medium in which the algal food is grown with crustacea may allow continuous cultures of herbivorous crustacea which are considered difficult to grow.