A laboratory colony of Hyalomma marginatum isaaci reared on rabbits in the adult stage and on guinea pigs in the immature stages, has suddenly shown a change of hosts, from a mammalian host to an avian host during its adult stage, and reverted back to a mammalian host in its immature stages. The change-over and reversai of hosts has been for a brief spell of one generation only. The next succeeding generation has shown the life cycle to proceed along the same pattern as already established. Possibilities of either host reversai and immunity or of preference changes within the tick during that particular one generation are suggested. The effect of such a change-over and reversai were it to occur in nature and the bearing such a phenomenon would have in the case of a tick-borne disease and a vector species of tick are envisaged.