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Although the occurrence of motion sickness is largely attributed to sensory conflict, the reasons for the highly variable inter-individual susceptibility to motion sickness remain unclear. One cause may be asymmetry between otoconial masses in the right and left inner ear. This hypothesis has been supported by experiments in fish under conditions of prolonged weightlessness and parabolic flight. In swordtail fish (Xiphophorus helleri) we examined the extent to which a Coriolis force environment, which has a strong motion sickness-triggering effect, disturbs swimming behaviour or produces motion sickness.In a sealed aquarium, fish (n=22) were subjected to Coriolis stimulation by constant vertical axis rotation, combined with pendular oscillation around a horizontal axis.Initially, at low stimulus intensity, all fish showed active compensatory swimming behaviour. However, while the majority (n=19) maintained active compensation movements at higher stimulus levels, the others (n=3) entered a passive uncoordinated state, from which they recovered again after reduction of stimulus intensity. On examination of the otoconial mass, we found asymmetries between the right and left utricles in all three cases and between the saccules in one case.We conclude that a lateral preponderance of the otoconial masses in the utricle or saccule promotes motion sickness under experimental conditions. The utricle appears to play a more important role in this than the saccule.