The morphology and development of Cowdria ruminantium have been studied in Amblyomma hebraeum and A. variegatum. Colonies of C. ruminantium have so far been demonstrated microscopically in gut, salivary gland cells, haemocytes and malphighian tubules of infected Amblyomma ticks. Colonies in gut cells were seen in both unfed and feeding ticks but colonies in salivary gland acini were observed only in nymphs that had fed for 4 days. Although the predominant type seen in both tick stages was the reticulated form that appeared to divide by binary fission, electron dense forms were also present. The latter are similar to those forms documented in endothelial cells of the vertebrate host as well as in cell culture. The presence of colonies of C. ruminantium in salivary glands of feeding ticks, along with the demonstration of different morphologic forms of the organism, suggests that a developmental cycle of the organism occurs in its invertebrate host. It is thought that organisms first infect and develop within gut cells. From there subsequent stages continue their development in haemolymph and salivary glands and are then transferred to the vertebrate host during tick feeding. Further studies are needed to completely understand the development of C. ruminantium in ticks and its subsequent transmission by these parasites.