The resistance rate of different populations of the same breed of creole Guadeloupean goats to Cowdria ruminantium infection varied greatly depending on the previous heartwater history of each population. After experimental infection of goats removed decades ago from endemic areas, the observed resistance rate was 25%, while it was 54% in a population that had been isolated from the disease for 10 years and reached 78% in a flock actually exposed to heartwater. This resistance seems to be under genetic control as sex and paternity were the most important factors which could explain resistance in a group of 90 kids of the same flock, tested under controlled conditions. Resistance rate varied greatly (20-83%) depending on the sire, with a heritability estimate of 0.49 for half sibs and 0.85 for full sibs. A recessive sex-linked gene could be involved in the genetic determination of this resistance. From these observations, it can be stated that in endemic heartwater areas, each population, i.e. each flock, will have developed resistance at a definite rate according to population, age and the extent of past and present exposure to the disease, through a natural selection of resistant lines. Populations removed from exposure to heartwater will progressively lose their ability to resist infection through an increase in the frequency of susceptible stock. If our hypothesis of a recessive sex-linked gene is proved correct, it should be easy to select for improved resistance to the Guadeloupe breed of goat to heartwater.