In persistently infected ISE6 cells, membrane proliferation was more extensive than in acutely infected tick cells; but, again, this was not as pronounced as in infected Vero cells (Fig. 7B) . Vesicles were again seen within proliferated, dilated ER and remained within the same size range as in acute infection. Virions were not noted in the samples we examined. One difference in the persistently infected ISE6 cells was particularly striking. Tubules were seen in almost all persistently infected ISE6 cells and often occurred in fascicle-like bundles of multiple tubules cloaked by a single membranous sheath (Fig. 7D ).