ABSTRACT A simple bioassay system was developed to study locomotory and ovipositional responses of screwworm, Cochliomyia hominivorax (Coquerel), flies to bovine blood inoculated with eight species of coliform bacteria that were isolated from screwworm-infested animal wounds. When exposed to odors from bacteria-inoculated blood which was incubated for 72 h at 37 degrees C, approximately 50% of 7- and 10-d-old gravid females landed on the blood by the end of 15 min test exposure. Only 17% of 7-d-old reproductively sterile females (from irradiated pupae) with previtellogenic ovaries and 2% of 4-d-old vitellogenic females responded to the same treatment. Females generally reacted in greatest numbers to bacteria-inoculated blood incubated for 72 h, followed by 48 h, then 24 and 96 h. Males of all ages tested were unresponsive. Although oviposition occurred in tests with gravid females lasting for 1 h, with both inoculated blood and an uninoculated control, the inoculated sample was significantly better than the control at 48, 72, and 96 h incubation duration. Our results are consistent with the conclusion that the inoculated blood, when incubated for 48-72 h, gives off volatile chemicals which attract gravid females and contains an oviposition stimulant that acts following contact and feeding. The volatiles, once isolated and identified, may be useful for sampling gravid females in the field as well as improving the oviposition system in the mass-production facility of the screwworm eradication program.