The development of Wuchereria bancrofti larvae within the mosquito host has been carefully worked out, but the migration of the larvae, once they have become infective, within the mosquito has been a matter of incidental observation. Further, there has apparently been no statistical study to verify an opinion held by some that larvae escape without the mosquito taking additional blood meals. Interest has been focused mainly upon the site of the escape of the larvae from the proboscis. Francis (1919) observed that the larvae, after a stay of approximately two weeks in the thoracic muscles, migrated to the proboscis. However, in his discussion of the factors limiting the spread of filariasis, he asserted that a large proportion of the larvae after they leave the thoracic muscles migrate into the abdomen and are lost. Yamada (1927) stated that it appeared to be certain that the majority of the larvae first found their way to the abdomen where they seemed to move about in the general body cavity for a short while and then took their course forward, traveling through the thorax and head to the proboscis. Hu (1933) wrote that the distribution of infective larvae within the mosquito suggested that their wanderings did not necessarily take them at once to the head; thus larvae which had escaped from the proboscis would be replaced by others coming up from the thorax and abdomen. Newton and Pratt (1945) demonstrated that infective larvae placed in the abdominal cavity of an uninfected mosquito migrated forward to the proboscis. Several workers have suggested that infective larvae escape from the mosquito host without the latter taking additional blood meals. Yamada and Komori (1926), while investigating the site of exit of the larvae, found that filarial larvae escaped from the labium by rupturing the tips of the labella when a stimulus such as pressure or heat was experimentally applied. They stated, in addition, that the larvae may also escape spontaneously in the same way when their hosts are fed on sugar-water solution. Hu (1935) examined 22 mosquitoes that had lived from 10 to 93 days following infection. He found infections which had lasted for a considerable length of time, even as long as 79 days after the incubation period of two weeks. However, he noted that his specimens kept after two weeks were held at a lower temperature and a lower relative humidity than those of the first two weeks. Furthermore, the 7 specimens examined through the first 16 days averaged 10 larvae per mosquito, whereas the 15 examined after this time averaged 3 larvae per mosquito; this indicated a definite loss. Galliard (1941) stated that at temperatures between 30 and 35? C the larvae do not remain in the mosquito longer than 9-10 days after they have reached the infective stage. In one experiment he found that the mosquitoes