1. The optomotor control of orientation and locomotion in the fruitfly Drosophila melanogaster requires the conveyance of information from distinct movement detectors in the visual system to distinct movement effectors in the motor system. Abnormalities of the optomotor control system have been found occasionally in Drosophila.
2. The abnormal flies can be isolated from population samples by appropriate fractionation according to the magnitude and the sign of the optomotor responses. A cyclically operating machine was used to fractionate two inbred strains, w+ and wα, which possess different alleles on the white-locus of their X-chromosomes.
3. Movements of an artificial visual environment elicit similar orientation-control responses, but antagonistic locomotion-control responses in the two strains. The responses depend on various parameters and may even change with habituation to the stimulus. However, the application of selection pressure through eight generations has little if any effect on the different optomotor behaviour of the inbred strains.