Listeria monocytogenes cells are ingested by protozoa such as Acanthamoeba sp. or Tetrahymena pyriformis. However, they are not killed, but survive within the protozoa and may multiply intracellularly. The protozoa are lysed within about 8 days, releasing viable L. monocytogenes. No co-existence was observed between L. monocytogenes and Tetrahymena. A co-culture of L. monocytogenes and Acanthamoeba sp. showed a decay of locomotive forms and release of listeria from vegetative protozoan cells whereas the bacteria were destroyed in cysts. These phenomena provide an insight into the pathogenesis of listeria infection in man and warmblooded animals because intracellular processes occurring in protozoa after ingestion of L. monocytogenes may be similar to those observed in mammalian cells.