Observations made while screening candidate attractants for the screw-worm fly, Cochliomyia hominivorax (Coquerel), revealed that light was an environmental factor affecting fly response. In unbaited traps about 25 times more flies were captured in light than in darkness. At temperatures ranging from 86°-113° F, 3 times as any flies were captured as at those from 67°-80°. Oviposition took place in temperatures ranging from 86°-113°; the greatest number of egg masses was deposited at 104°±6°. In baited traps very few 1- to 2-day-old flies were captured among 1- to 7-day-old flies tested; the greatest attraction was obtained when the flies were 3-4 days old. Among flies 6–8 days old tested for their attraction to liver and blood. about 5 times more females than males were trapped.