An electron microscopic study of structures suspected to represent a possible developmental cycle of Cowdria ruminantium in reticulo-endothelial cells of mice and ruminants is reported. After infection dense bodies increase in size and undergo division to form fragmented dense bodies. These in turn apparently sub-divide and become organized to give rise to mature "organisms". In none of these structures do limiting membranes separate the parasitic inclusions from the host cell cytoplasm. Present observations suggest that growth of the organism in reticulo-endothelial cells differs from that of chlamydial and rickettsial agents and somewhat resembles the replication of some viruses. Developmental stages observed after infection of ruminants with the Ball 3 strain of the heartwater agent are indistinguishable from those seen with the mouse adapted strain. These observations support the hypothesis that C. ruminantium released from reticulo-endothelial cells subsequently penetrates endothelial cells where further multiplication by binary fission occurs.