ABSTRACT Removal of potential pathogenic bacteria, for example, during wastewater treatment, is effected by sorption, filtration, natural die-off, lysis by viruses, and grazing by protists, but the actual contribution of grazing has never been assessed quantitatively. A methodical approach for analyzing the grazing of protists on ¹³ C-labeled prey bacteria was developed which enables mass balances of the carbon turnover to be drawn, including yield estimation. Model experiments for validating the approach were performed in closed microcosms with the ciliate Uronema sp. and ¹³ C-labeled Escherichia coli as model prey. The transfer of bacterial ¹³ C into grazing protist biomass was investigated by fatty acid (FA) and RNA stable isotope probing (SIP). Uronema sp. showed ingestion rates of ∼390 bacteria protist ⁻¹ h ⁻¹ , and the temporal patterns of ¹³ C assimilation from the prey bacteria to the protist FA were identified. Nine fatty acids specific for Uronema sp. were found (20:0, i20:0, 22:0, 24:0, 20:1ω9c, 20:1ω9t, 22:1ω9c, 22:1ω9t, and 24:1). Four of these fatty acids (22:0, 20:1ω9t, 22:1ω9c, and 22:1ω9t) were enriched very rapidly after 3 h, indicating grazing on bacteria without concomitant cell division. Other fatty acids (20:0, i20:0, and 20:1ω9c) were found to be indicative of growth with cell division. The fatty acids were found to be labeled with a percentage of labeled carbon (atoms percent [atom%]) up to 50. Eighteen percent of the E. coli -derived ¹³ C was incorporated into Uronema biomass, whereas 11% was mineralized. Around 5 mol bacterial carbon was necessary in order to produce 1 mol protist carbon ( y _{x / s} ≈ 0.2), and the temporal pattern of ¹³ C labeling of protist rRNA was also shown. A consumption of around 1,000 prey bacteria (∼98 atom% ¹³ C) per protist cell appears to be sufficient to provide detectable amounts of label in the protist RNA. The large shift in the buoyant density fraction of ¹³ C-labeled protist RNA demonstrated a high incorporation of ¹³ C, and reverse transcription-real-time PCR (RT-qPCR) confirmed that protist rRNA increasingly dominated in the heavy RNA fraction.