In the course of our investigations on the biology of flies, during the year 1912, a certain number of experiments were performed in order to ascertain whether house-flies possess any colour preference. In the case of mosquitoes, Nuttall (1901) has shown that these insects have a very well-defined preference for certain colours. When a number of boxes, lined with different coloured materials, were placed in a tent containing mosquitoes, the latter occurred most frequently on navy-blue, and, in descending order, on dark red, brown, scarlet, black, slate-grey, dark green, violet, leaf-green, blue, pearl-grey, pale green, light blue, ochre, white, orange, and yellow. Very few insects indeed were found to rest on the last seven colours.