Ecological stoichiometry suggests that herbivore growth is limited by phosphorus when this element in the diet is < 8.6 µgP mg C-1 (C:P atomic ratio >300). However, in nature, it is not necessarily related to the relative phosphorus content in diets. This may be the result of complex feeding and assimilation responses to diets. We examined these possibilities using herbivorous plankton fed mono-specific and mixed algae varying in phosphorus content of 1.6 to 8.1 µgP mg C-1. The herbivores showed a 10-fold growth rate difference among the diets. Growth rates related poorly with phosphorus content in the diets (r2=0.07), better with P ingestion rate (r2=0.41) and best with phosphorus assimilation rate (r2=0.69). Inclusion of assimilation rates for carbon and fatty acids increased 7% of the explained growth variance. These results indicate that the feeding and assimilation flexibilities play pivotal roles in acquiring a deficient element and in regulating growth rate.