In the summer and early autumn of 1974, flies were trapped in the Beirut districts of Ashraffiyah, Burj, Burj-Hammoud, Quarantine, the New Slaughter House, Ras Beirut, American University Campus, and the suburbs of Chiyah and Mreyjieh. Musca domestica, Muscina stabulans, Calliphora vicina, Phormia regina, Phaenicia sericata, Sarcophagidae spp. and Fannia canicularis were identified. The rural areas of Mreyjieh and Chiyah had higher densities of Musca domestica while higher densities of Calliphoridae spp. were found in the Slaughter House, Quarantine and Burj-Hammoud districts. Twenty-nine isolates, ten of Shigella spp. and 19 of Salmonella spp. were obtained from the bacteriological studies of 72 batches of Muscidae spp. and 84 Calliphoridae spp. These results indicate the hyperendemicity of salmonellosis and shigellosis in the areas under study, due to the prevalent poor sanity conditions and high fly infestation. It is suggested that in countries where valid health statistics are not available, bacteriological studies of flies may provide essential epidemiological information.