DARCY GILMOUR, D. F. Waterhouse and G. A. Mclntyre, in a paper entitled ”An Account of Experinients undertaken to Determine the Natural Population Density of the Sheep Blowfly, Ludlia cuprink Wledl.”, have endeavoured to assess the value of trapping as a means of controlling those insects (Ommonwealth Coun. Sei. Ind. Research, Bull. 195; 1946). The method used was that of liberating a known number of marked flies and of sampling by means of traps the population in an area surrounding the point of release. The number of blowflies within the area was then calculated by multiplying the ratio of the unmarked to marked flies caught in the traps by the number of marked flies liberated in the trapping area. Some 102 traps were used, and these were disposed at equal intervals in a circle of 6 miles diameter. Some 40,000 flies were liberated at the centre of this circle one day before the trapping began. The marking of the blowflies was by staining them with an alcoholic solution of suitable dyes. Two treatments were given: the first with an electric power sprayer and the second with a hand atomizer. In the main experiments, the diameter of the circle was increased to 8 miles.