The splicing factor RBFOX1 exerts a pleiotropic effect on numerous psychiatric and neurodevelopmental disorders; however, the exact working mechanisms remain largely unknown. One potential mechanism that could partially explain the influence of RBFOX1 on some psychiatric traits is through the modulation of the serotonergic system, a common target for medical intervention in psychiatric disorders associated with RBFOX1.Here we studied a rbfox1 mutant zebrafish model, carrying a rbfox1sa15940 allele that expresses a shorter, aberrant rbfox1 mRNA. In the homozygous rbfox1sa15940 zebrafish we observed a significant reduction of serotonin levels in the telencephalon and diencephalon. Additionally, acute administration of fluoxetine partially reversed some of the associated behavioural alterations. The hyperactivity and altered shoaling behaviour observed in the rbfox1sa15940/sa15940 zebrafish were reversed with fluoxetine treatment. However, in other paradigms, hyperactive behaviour persisted, suggesting a distinct intrinsic motivation for locomotion in different contexts. Interestingly, alterations found in aggression and social novelty tests were not reversed with fluoxetine treatment, suggesting the involvement of other neurological mechanisms in these behaviours. These findings underscore the importance of understanding the complex working mechanisms of RBFOX1 in psychiatric and neurodevelopmental traits to gain better insight into related traits and their pharmacological treatment.