The technique of isolating and identifying yellow fewer virus on cells of Aedas aegypti, clone C17, is more rapid than the methods of intracerebral inoculation in the baby mouse or intrathoracic inoculation in the Toxorhinchites mosquito; the diagnosis can be made in 3 days, whereas with the other techniques it takes about a fortnight. These third techniques were used to isolate 6 strains o( yellow fever virus : 4 from human blood and 2 from Aedes furcifer. The results of direct examination of the heads of these two mosquitos by indirect immunofluorescencc were negative. These non-adapted virus strains develop poorly in cultures of mammalian cells of the Vero type, and this contraindicates their use for obtaining a rapid diagnosis of yellow fever.