Blood from dogs containing large numbers (510–1160 per 20 mm 3) of D. immitis microfilariae was inoculated into grown caterpillars of the bee-wax moth, Galleria mellonella. Development of D. immitis was observed in the hemolymph and adjacent organs of caterpillars kept at room temperature (26–30 °C). Of 334 caterpillars dissected, 153 showed developmental forms (44.9%). Growth and morphological changes in microfilariae were first observed at the end of the third day after blood inoculation into caterpillars. By the end of the fourth day “sausage” forms predominated. Growth of D. immitis is halted usually at the “sausage” stage. Such viable forms may, however, survive in caterpillars kept at room temperature for periods up to 16 days. No development of D. immitis was observed at 37 °, although active viable microfilariae may survive in their experimental hosts up to 8 days. Cellular immune reactions: chitinization of parasites, encapsulation, giant cell and pseudo-tubercle formations around D. immitis were seen in caterpillars inoculated with blood containing microfilariae. The results of the present investigation are discussed and evaluated in the light of comparative studies of experimental infections of D. immitis in different mosquito species—their natural intermediate hosts.