Grazing experiments with the calanoid Calamoecia lucasi and the cladoceran Ceriodaphnia dubia feeding on free lake bacterioplankton produced artificially high clearance rates by the acridine orange direct count method (AODC), but lower and more conservative clearance rates by labelling bacteria with [3H]thymidine. A comparison of zooplankton carbon requirements and bacterial biomass suggests that even at maximal filtration rates bacterioplankton can only be a minor constituent of the diet. Bacterioplankton and phytoplankton do not provide adequate carbon to satisfy the daily requirement for respiration let alone for growth and reproduction. This suggests that: (i) predation of small zooplankton occurs; (ii) stress depresses the clearance rates; or (iii) the values obtained are at the low end of the diel range of feeding rates.