The effects of four diets (Chlamydomonas reinhardtii, Selenastrum capricornutum, yeast-trout chow-Cerophyl (YTC) and YTC plus S. capricornutum) and two reconstituted waters on the vitality and fecundity of five to six life-span generations of Ceriodaphnia dubia were determined. The foods and waters were tested with and without a selenium supplement. The results indicate that healthy populations of this species can be maintained in reconstituted waters containing only four salts so long as the food is nutritionally adequate and the reconstituted water is formulated from an ultrapure base water. The adequacy of the YTC diet varies according to the nutritional adequacy of the constituents. Some batches of trout chow and Cerophyl were nutritionally deficient, causing unacceptably high mortality and abortions in cultures. These deficiencies were corrected by adding selenium to the culture water and using Selenastrum in conjunction with the YTC. The alga C. reinhardtii, if cultured in medium enriched with vitamins and selenium, proved quite adequate for maintaining healthy cultures of C. dubia. Animals maintained on the other alga, S. capricornutum, exhibited a delay in reproductive maturation and a reduction in brood sizes, and they were frequently trapped in the surface film of the culture water. (Copyright (c) 1989 Setac.)