The demonstration by Bennett and his colleagues1 of the Harvard Medical School that the common New Jersey mosquito is capable of transmitting yellow fever to monkeys seems to throw doubt on the conventional assumptions for other insect-borne diseases. Following experimental proof by Reed, Carroll and their co-workers in 1900-1901 that the subtropical mosquito variously known as Aedes aegypti, Stegomyia fasciata or Aedes argentens is capable of transmitting yellow fever to man, it was quite generally assumed by hygienists that this is the only species of mosquito capable of transmitting this infection. It was alleged in support of this assumption that the geographic distributions of Aedes aegypti and yellow fever are the same. Not till a quarter of a century later, after Stokes, Bower and Hudson (1928) had shown that rhesus monkeys are susceptible to yellow fever, was it practicable to make adequate tests of other species. Two other subtropical species of mosquitoes were then shown to be equally efficient vectors of yellow fever, while a third subtropical species would occasionally transmit the disease in an attenuated form.