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Based on a long-term study of many years, a report is given on the behaviour of Coxiella burneti in the argasid tick, Ornithodoros moubata. Recent observations were made on ticks infected on mice and subsequently maintained separately. Particular emphasis was placed upon the localization of the ricksettsiae in certain organs of the ticks, its excretion -also with regard to routes of transmission - and transovarial passage. C. burneti invade primarily the gut epithelial cells of the tick and these cells remain infested with the causative agent throughout the entire life of the vector. After a certain infection period, other organs of the tick may become also infected. Thus, infection of the coxal organ, of salivary glands, rectal ampullae and ovaries has been confirmed through the presence of the rickettsiae in the coxal fluid, saliva and excreta or through transovarial passage of the agent with the saliva during feeding transmission to a new host takes place. Infection of the various organs of the tick and excretion or passage of the agent is not necessarily the rule, but may be even considered as an exception. This is especially the case for transovarial passage. On the other side, the multiplication of rickettsiae in the tick can be so intensive that the haemolymph and all internal organs become flooded with the organisms. These observations have been made in moribund and dead ticks. One of the most remarkable results was that the behaviour of C. burneti in its tick host varied considerably not only within the same series of experiments but more frequently in one and the same individual tick as well. Thus, e.g. excretion of the agent - with the saliva or coxal fluid - could be interrupted and resumed again later on. Evidently, these variations indicate a shift in the host-parasite interrelationship in which the multiplication of the rickettsiae is greatly enhanced by a decrease in the host's immune response, whereas an increase in its defence reaction will cause suppression of multiplication. It is not known, however, if the relevant impulse is primarily given by the tick or the rickettsial organism. The behaviour of C. burneti in O. moubata which does not follow a definite pattern coincides well with the high adaptability and variability characterizing this microorganism in other aspects as well.