In a previous paper on Bird Trypanosomes it was pointed out that these organisms grew readily in the test-tube on blood agar and that the resulting forms resembled the flagellates which Schaudinn found in the gut of mosquitoes which had fed on owls infected with Halteridium and with H. Ziemanni. In other words, the position taken was that the flagellates observed in the mosquitoes did not represent stages in the life-history of intracellular parasites but were actually cultures in vivo of trypanosomes present in the blood of the birds used. In confirmation of this position it was desirable to show that trypanosomes could actually grow and multiply in the gut of mosquitoes and that such forms actually did correspond to those which would be obtained in vitro.