We report on an observational study on sex differences in serious and playful aggression in early childhood. The sample included 14 girls and 14 boys, aged 2 to 4. The study was carried out in Porto Alegre, Brazil. Half of the subjects attended a nursery school for children from a favela (slum district); the others attended a nursery school that was run by a local university and represented the life conditions of the middle class of the Brazilian society. Each of the children was observed at school for 3 hours (divided into 12–15 minutes episodes) through a focal sampling technique. Significant sex differences are found for the frequency of playful aggression for both the actor's and the “victim's” part of the aggressive act, but for the actor's role the effect holds only for the middle class setting. There are also tremendous differences between the frequencies of serious aggression in girls and some of the boys, although the effect is not statistically significant. When several types of aggression are compared, it becomes evident that sex differences are restricted to bullying (dominant aggression). The rates of instrumental and reactive aggression are similar in both sexes. Girls and boys from the favela setting do not differ in the relative importance of the different types of serious aggression, but in middle class children the proportion of acts of bullying is much higher in boys than in girls.