Abstract In a cylindrical drum (180° black, 180° white) the direction of spontaneously running de-winged flies Drosophila melanogaster white, white-apricot, sepia , and +, and Drosophila subobscura light red, cherry, poppy-red, plum, maroon , and + toward the black and white areas was measured. In analysing these directions, the position of each fly was determined to the nearest 10° at four different distances from the centre of the drum. The preference for the black area and the contrast-line can be correlated to the content of screening pigments in the compound eyes. Especially in Drosophila melanogaster white, white-apricot , +, and sepia a ratio of 1·0 : 1·4 : 2·8 : 3·5 is found for the stimulus efficiency of the black area including the contrast-line; for the mean orientation times one gets a ratio of 17 : 5 : 1 : 1. These values correspond exactly to the amounts of light received by the receptors of the mutants and the wild type in equally illuminated eyes. In the super-pigmented mutants sepia of Drosophila melanogaster and maroon of Drosophila subobscura the maximum of the distribution curve does not coincide with the contour but is shifted by 10° to the black area.