IN an article on "Activation of Pyrethrins in Fly-Sprays", David and Bracey1 state that the test insect used in their work was the mosquito, Aedes aegypti, and point out that their findings may not hold for other insects. Nevertheless, the use of the term "fly-sprays" in the title is perhaps unfortunate, for we have found that many of their conclusions drawn from work on Aedes do not hold when the house-fly, Musca domestica, is the test insect. As shown below, the substitution of lubricating oil, etc., for sesame oil in fly sprays, or in dual-purpose sprays intended for fly and mosquito control, would be unjustified, since such substances do not increase the toxicity of pyrethrins to flies.