Fathead minnow (FM, Pimephales promelas) are a species of small fish native to North America. Their small size, fast development, and ability to breed in the lab make them an ideal species to use in research, especially in toxicology. Behaviour in general is poorly studied in FM. The aim of this study was to characterize the normal behaviour of fathead minnow at 3 different stages of development in a light-dark box and in a social behaviour test. Fish larvae showed a preference for the light area, and then an increase in dark preference was seen as the fish aged. FM preferred to be with conspecifics at each age, but this preference was much stronger at the adult stage. The time of first entry into the conspecific area was reduced with increasing age of the fish. The time spent in the conspecific area increased between the juvenile and adult stage, and adults stayed more in this area when they entered it. Maturation of behavior in FM was demonstrated in our study. The FM is another good model fish to assess behavioral effects of chemicals, and this study helps to define the appropriate ages for behavioral studies with FM.