Houseflies (Musca domestica) are mechanical vectors of Shigella organisms. Seasons during which both flies and cases of dysentery are prevalent often coincide. Houseflies have an affinity for human excrement and, as documented by bacteriologic culture of trapped flies, become contaminated with Shigella organisms following contact with infected human feces. The inoculum required to transmit shigellosis is only 10-10(2). Since houseflies cohabit with humans, they can readily contaminate food and eating utensils. With the advent of insecticides in the 1940s, intervention studies in the United States provided evidence of the role of the housefly in transmission of shigellae. In towns that were exposed to fly-control measures, the density of flies and the prevalence of carriage of Shigella organisms, diarrhea, and mortality due to diarrheal disease among young children all markedly diminished as compared with the situation in control towns. Investigations that quantitate the importance of fly-borne transmission of Shigella organisms relative to other modes of transmission and that provide evaluation of measures to diminish fly-borne transmission are warranted in developing areas.