We examined population growth rate, reproduction and phosphatase activity in digestive tracts of individual Brachionus calyciflorus fed by P-replete (molar C:P = 107) or P-depleted (C:P = 920) algal food supplied at three carbon (C) concentrations (1, 2, and 4 mg C L-1). Rotifer growth rate was significantly influenced by P content, whereas the effect of algal C concentration was not significant. Both algal P content and C concentration had a significant effect on the egg ratio. The individual amount of chlorophyll a (chl-a) was considered as a proxy for the ingested algal food occurring in the rotifer’s gut and phosphatase activity in the gut likely reflected variable individual assimilation of P under different food treatments. The amount of both chl-a and phosphatase activity in the rotifers fed by P-replete algae significantly increased with an increase in C concentration in the food suspension. In contrast, the amount of chl-a significantly decreased with an increase in food concentration, and phosphatase activity did not change significantly over different food quantity in the rotifers fed by P-depleted algae. Thus, the hypothesis that P-deficient rotifers use preabsorptive regulation by changing activity of the digestive enzymes is not supported by the data.