During the period April 13 to November 10, 1953 the writer conducted a survey of calyptrate flies of medical importance at London, Ontario, and its vicinity. The flies were trapped in twelve traps baited wit11 malt extract and set out in twelve localities such as garbage dumps, the border of swamps and the premises of meat packing plants, sewage disposal plants, dairies, restaurants and industrial properties. During the course of the survey 142,975 flies in the families Anthomyiidae, Muscidae, Calliphoridae and Sarcophagidae were collected. From these were chosen twenty species for counting. The daily catch of these flies from each of the twelve traps was kept separate and the flies were sorted according to species and sex and a record was kept of the numbers of each species captured in each trap. Tn making the identifications it was necessary to examine each specimen under a binocular microscope and during this process the specimen was also examined for the presence on it of parasites or other organisms clinging to it. Three such organisms mere found on several flies: mites, pollinia of milkweed and fungi of the genus Empusa.