The effect of the chemical composition of food on the reproductive success of the copepods Acartia tonsa Dana and A. hudsonica Pinhey was studied in the laboratory. Laboratory-reared individuals were fed one of three monoalgal diets at different stages of growth: the diatom Thalassiosira weissflogii, the flagellate Rhodomonas lens and the dinoflagellate Prorocentrum minimum. The diet was analyzed for carbon, nitrogen, protein, carbohydrate and fatty acid content. Reproductive success was measured as eggs female(-1) day(-1) (E(r)) and as the hatching success of the eggs. The E(r) of Acartia spp. was correlated with protein and specific fatty acids [16:1 omega 7 (negative), 20:5 omega 3, 22:6 omega 3, and 18:0 (positive)] and, especially, the fatty acid composition of the algae expressed as the omega 3:omega 6 and 20:22 fatty acid ratios. The youngest diatom cultures and exponentially-growing flagellates displayed the highest E(r); the lowest E(r) was recorded for females fed the senescent diatom cultures. The development time of eggs was affected by the age of the phytoplankton culture fed to the female. Hatching success of eggs decreased with the age of the algal culture, but no correlation was found with the measured chemical components of the food.