We investigated the process of digestion and estimated ingestion from digestion rate and food vacuole content in a planktonic freshwater bacterivore. Digestion was examined in feeding and nonfeeding Bodo flagellates previously exposed to Synechococcus and in ``naive'' flagellates. Digestion of Synechococcus by Bodo was estimated for flagellates in different growth phases. Food vacuole contents declined exponentially, and digestion rate was relatively constant, averaging about 1% food vacuole content min(-1) at 22 degrees C, regardless of feeding history or growth phase. There was close agreement among ingestion rates based on digestion rate and rates estimated from the disappearance or direct uptake of Synechococcus. We also examined ingestion of Synechococcus by Bodo. Ingestion was similar in cells of different growth phases and in the presence or absence of fluorescent microspheres. In contrast, in the presence of Synechococcus, microspheres were ingested at lower rates in late-exponential-phase cells and not at all in mid- and late-stationary-phase flagellates. Results of a preliminary experiment with marine Synechococcus and mixed marine flagellates gave results similar to those obtained with Bodo in terms of an exponential decline in average cell contents and a digestion rate of about 1% cell contents h(-1). Our results suggest that food vacuole content can be used to estimate ingestion of autotrophic picoplankton by heterotrophic nanoflagellates.