Visual flight control is studied in three mutant alleles of the gene ellipsoid body open (ebo) of Drosophila melanogaster. In mutant ebo flies the central complex is disturbed to varying degrees. Defects range from a small opening in the ellipsoid body to the dissociation of the ring into two parts, a cleft in the fan-shaped body and hypoplasia in the protocerebral bridge. Other parts of the brain are not visibly affected. Flight behavior is normal with respect to the amplitude of the optomotor response and to the object response (single rotating stripe). A reduced amplitude in the small random oscillations of the torque trace (yaw torque activity), however, is found in all three alleles. In two of them the frequency of torque spikes is reduced. In the allele ebo678 the dynamics of the optomotor response is altered. Upon reversal of the direction of rotation mutant flies take longer than wild type to shift their yaw torque to the new response level (optomotor reversal time). Finally, these flies also behave abnormally in the flight simulator in which their yaw torque controls the angular velocity of the panorama. Many ebo678 flies fixate a single stripe less persistently than normal flies, some even trying to fly away from it (antifixation). In ebo678 gynandromorphs the four behavioral phenotypes ("yaw torque activity", "torque spike frequency", "on-target fixation" and "optomotor reversal time") are all highly correlated with the phenotype of the ellipsoid body. Yaw torque activity and torque spike frequency in addition are correlated with the phenotype of the thorax suggesting that these behavioral defects are in part caused by mutant influences on the ventral ganglion. The results support the hypothesis that the central complex is involved in the control of flight behavior.