Titrations were made of yellow fever virus in stegomyia mosquitoes, using rhesus monkeys as test animals. It was found that: (a) The average mosquito immediately after engorging on highly infectious blood contained between 1 and 2 million lethal doses of virus. The titer of freshly ingested blood was as high as 1 billion lethal doses of virus per cubic centimeter. (b) During the fortnight succeeding a meal on infectious blood there occurred a reduction of titratable virus to not more than 1 per cent of that present in the freshly fed insects. (c) The titer was somewhat higher at later periods. This rise in titer signified possibly not a multiplication, but merely an increase of extracellular virus and of that easily freed by grinding to a titratable form. (d) At no later stage did the quantity of titratable virus equal that demonstrable in freshly fed insects.