Interaction between T. vaginalis and P. aeruginosa was studied in vitro. Protozoan cells were incubated in 0.9% NaCl at 37 degrees C for 30 and 150 min. with bacteria, alive or killed by heating for 10 min. at 100 degrees C. Transmission electron microscope observations showed weak ++phagocytic activity (less than 10%) of T. vaginalis against living P. aeruginosa. Phagocytosis of these bacteria caused death of T. vaginalis. Disintegration and autolysis of T. vaginalis resulted probably from action of exotoxin A produced by P. aeruginosa. Phagocytosis of dead P. aeruginosa has not been observed.