The experimental results show clearly, that low temperatures have a delaying effect upon the development of the free-living stages of B. decoloratus, B. microplus and M. winthemi. The tolerance of eggs to low temperatures is greatest in M. winthemi, less and approximately equal in B. decoloratus and B. microplus. The percentage of larval hatchings is greatest in M. winthemi, considerably less and approximately equal in the two boophilids. Larvae of M. winthemi are most tolerant of low temperatures, B. decoloratus occupies an intermediate position, B. microplus being the most susceptible. The experimental findings in conjunction with the geographical di stribution indicate that cold is not a limiting factor in the spread of M. winthemi, that it exerts a slight influence on B. decoloratus by limiting its numbers, and that it undoubtedly plays a role in restricting the spread and survival of B. microplus.