To further substantiate the use of mecp2−/− zebrafish as a model for Rett syndrome and to investigate basic visual function, we measured general locomotion and anxiety-related behaviours at three different ages [5 dpf, 21 dpf, and 6-months of age], PTZ-induced hyperlocomotion in larval 5 dpf fish, social preference for vision-based social stimuli when zebrafish start shoaling [21 dpf] and social, anxiety, and locomotion behaviour and cortisol levels in adulthood in mecp2−/− zebrafish. The gene discussed is MECP2; the disease is atypical Rett syndrome.