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Abstract: The catfish Synodontis nigriventris often shows a unique swimming behaviour in being oriented upside-down. When swimming near a (e.g., vertical) substrate, however, the animals orient themselves with their ventral side towards this substrate. This tendency is called ventral substrate response (VSR). The VSR does not only override the upside-down swimming behaviour but also the dorsal light response and the ventral light response. In the course of an earlier drop-tower experiment performed at ZARM (Bremen, Germany) using cichlid fish (Oreochromis mossambicus), we had observed that about 90% of the animals revealed sensorimotor disorders (kinetotic swimming) due to the almost complete lack of gravity as a cue for orientation. In order to further assess the importance of the VSR for postural control in S. nigriventris when being located near a substrate, we subjected catfish in relatively small chambers to drop-tower flights. In contrast to our results regarding cichlid fish, S. nigriventris showed no kinetotic behaviour. This clearly suggests that the VSR overrides even vestibular input and possibly represents the most important single behavioural response in this species. [Copyright &y& Elsevier]