Ileal symbiont intracellularis (ISI) isolated from Australian cases of PIA and PHE was replicated in the rat ileum enterocyte cell line IEC 18. The number of ISI within cells varied, as did the number of ISI infected within the monolayer. At 24 h post infection a large number of cells were infected with approximately 100 ISI per cell. At the termination of infection, fewer IEC 18 cells were infected but ISI had replicated to fill the cell cytoplasmic space. Numerous foci of infected cells were visible in the monolayer, containing as many as 15 densely infected cells. Division of ISI infected cells indicated the transmission of ISI in the cytoplasm to daughter cells. This suggests that the replication of ISI in culture appears to be reasonably cell dependent. No cytopathic effects were observed in the infected cultures.