ABSTRACTTetrahymena pyriformisingestedEscherichia colifor 15–20 min and the fine structure of food vacuoles was analyzed 5, 15, 30, 60, 90, 120, and 180 min after uptake began. From this analysis, eight vacuolar stages could be defined, and three to four stages were found in each sample.Stage 1represents forming and newly detached vacuoles with a random distribution of bacteria.Stage 2is the “dehydration” vacuole in which the bacteria are compacted and a few may lyse.Stage 3, corresponding to the acid phosphatase‐positive stage, has an electron‐dense vacuolar matrix revealing components of lysed bacteria and the translucent coat of intact bacteria.Stage 4is the “halo” stage where centrally located, intact bacteria are surrounded by lysed material being removed by pinocytic activity of the vacuolar membrane.Stage 5represents lysis of bacteria remaining intact until this stage; the stage is apparently followed by a second stage 4.Stage 6contains few bacterial profiles in a smeared homogeneous mass.Stage 7contains numerous vesicular membranous structures which apparently become transferred to the cytoplasm as such.Stage 8represents defecation vacuoles derived from fusion of smaller vacuoles. The main findings are as follows: I) Bacterial lysis may occur during acidification of the vacuole prior to fusion with lysosomes. II) Digestion of bacteria apparently occurs in “bursts” as indicated by the extended time that vacuoles in stages 4 and 5 are present. III) Bacterial membranous structures seem to be transferred directly to the cytoplasm ofTetrahymena. IV) Mass defecation occurs 2 h af