Abstract.Growth rates and fecundity ofBrachionus plicatilisfed baker's yeast, a diatom, and three different flagellates, were monitored during a 15‐day feeding trial. Identical growing conditions and equivalent food levels for the different diets were maintained during rotifer culture. The highest growth rate and mean fecundity were exhibited by rotifers fedChlorella saccharophila, followed in decreasing order by those fedIsochrysis galbana(T‐iso),Tetraselmis suecica, Saccharomyces cerevisiaeandThalassiosira pseudonana. Significant differences in contents of neutral lipids, polar lipids, sugars, glycogen, protein, ash and corresponding energy reserves in the resultant five groups of rotifers were diet dependent. Total available energy, about 15 kJ/g in rotifers fedS. cerevisiaeandI. galbana(T‐iso) was significantly different from the 14kJ/g in rotifers fedT. suecicaand C saccharophila, and in turn from the 13kJ/g in rotifers fedT. pseud