We examined the hypothesis that different types of ingested matter may be processed at different rates by a marine microzooplankter. Ciliates were exposed to a prey item and then diluted at 1:100. Disappearance or digestion of inert (fluorescent microspheres) and digestible prey items (heat-killed and fluorescently labeled Synechococcus, natural Synechococcus, and Isochrysis galbana) were determined from changes in cell contents using epifluorescence microscopy. Ingested prey declined exponentially, and prey analogues were processed like natural prey. There were no significant differences in rates of disappearance or decay constants for the different prey items or in feeding vs. nonfeeding ciliates or between ciliates cultured on (or naive to) natural Synechococcus. The overall average t(1/2) of cell contents at 22 degrees C was 75 min. The exponential declines in cell contents indicated that some ingested chlorophyll a, whether in Synechococcus or Isochrysis, can be long-lived. The relatively invariant decay constant for ingested matter opens the possibility of using a gut passage approach to estimate instantaneous rates of grazing for natural populations of oligotrich ciliates.