The aim of this research was to investigate the differences in aggressive and prosocial behavior in preschool children in relation to age and gender. 107 preschool teachers from different districts of Croatia participated in the survey. The educators assessed the aggressive and prosocial behavior of a child from the educational group using the Scale for the Assessment of Aggressiveness and Prosocial Behavior in Children (Žužul and Vlahović-Štetić, 1992). The test results are partially consistent with previous research. Namely, it has been confirmed that aggressiveness is more common in nursery children than in kindergarten-age children. The reason for this is that older children are cognitively and emotionally more developed and are able to express their feelings in a socially acceptable way. Kindergarten-age children, on the other hand, are not yet able to express their feelings verbally or emotionally as they should. This leads to frustration, which often manifests itself in tantrums. This is precisely why aggression is more pronounced in younger children. When analyzing the results, gender differences in aggression were also found. Boys show aggression more often than girls. As for prosocial behavior, the survey confirmed that it increases with age, i.e. it is more pronounced in children of kindergarten age. According to the survey, the older children are, the greater their need to help someone and comfort them if necessary, and they feel compassion for that person. Previous research has found that prosocial behavior is more pronounced in girls than in boys. However, this research did not confirm a significant gender difference in prosocial behavior in preschool children. Research has established that aggressive and prosocial behavior are interrelated. In fact, it is evident that the level of aggressiveness in children decreases when the level of prosocial behavior increases and vice versa.