Summary Intracellular symbionts have long been studied in insects but only within the past few years in blood-sucking forms. They were found first in tsetse flies, then in other species of Diptera, in lice, in bedbugs, and finally in a few other blood-sucking invertebrates. Sikora and Buchner, working independently, found them in hog lice in 1919 and Hindle in 1921. We saw them in 1919 but did not recognize them as such until later, when a search was being made for Rickettsia bodies. They resemble the symbiotic fungi of other insects. They are Gram-negative. We have not succeeded in cultivating them either in hanging drops or in culture tubes. They live in enlarged epithelial cells, called mycetocytes, situated on the wall of the mid-intestine and are present in young and mature lice. They escape into the intestine by the rupture of mycetocytes, and it is still undetermined whether or not they infect other epithelial cells at this time. The symbiont is transmitted through the egg, in which it lies at the posterior pole in a small inpocketing of the vitelline membrane and between it and the shell. In the adult female there is a mycetome in both oviducts, and the hypothesis is suggested that the symbiont on escaping from the mycetome grows up the stalk of the follicle and reaches the egg through the delicate canals of the “eistigma” at an early stage in the development of the latter. The term symbiont was first used by Koch, because he believed that they were connected with some physiological function, but it is still undetermined whether they represent a real case of symbiosis. The finding of symbionts in a number of forms whose only resemblance is that they live exclusively on the blood of vertebrates, and the fact that the symbionts are in most instances to be found in the region of the mid-intestine has led to the belief that they are in some way connected with the physiology of digestion. The evidence of our results in favor of this view rests on four facts, namely, (a) the location of the mycetocytes in the mid-intestine where digestion takes place, (b) the mechanical control of the increase of the symbiont through the rupture of the mycetocytes, (c) the careful provision for transmission to the next generation, and (d) the high mortality and the inability to raise a second generation when the lice are removed from their natural host and fed on man.