The planktonic crustacean Daphnia magna was fed varying quantities or quality (C:N:P ratio) of phytoplankton in a flow-through chemostat culture. Size-specific dry weight (DW) and egg production varied with a factor of >3, mainly governed by food quantity. A maximum was reached at ∼4 mg C/l. Mean C/DW was 43.96 ± 4.43% and insignificant variations were recorded between the groups of fed animals, but a decrease to 38.09 was found in starved animals. Mean N/DW was 8.18 ± 1.02% with small variation between the treatments, and again with the lowest value in starved individuals. The mean P/DW of 1.38 ± 0.3% of DW, was largely unaffected by food quantity or the absolute and relative supply of paniculate P in the food. The results indicate a rather constant element/DW ratio in Daphnia within normal ranges of food supply. This has strong implications for biomass determinations, and is in particular important when modelling elemental turnover and release from Daphnia.