Perhaps due to the increase in the workforce of mothers with young children, much attention has been directed in the past decade towards the subject of out-of-home child care.In order to gain further understanding of the socialization of preschool children, an observational study was carried out which was designed to measure the amount of negative behavior expressed by children participating in mixed-age versus same-age preschool groups.An observational code was developed to test the 2 hypothesis that three-to five-year-old children in a same-age group would exhibit more negative behavior than would those in a mixed-age group.Sixteen children in each group (matched for age, sex, and number of hours in school per week) were observed for a total of sixty minutes each. All children included in the study had been enrolled in the facility used for at least eight months.Analysis of variance determined no significant group effect, indicating that there was no difference in the negative behavior of children in the same-versus mixed-age groups.However, the Rank-Sum test revealed a striking gender effect, indicating that the frequency of negative behavior exhibited by boys in this sample was much greater than that of the girls.These findings are consistent with those of previous researchers, and clearly demonstrate a gender difference in rates of aggression for children as young as three.Implications for gender and age groupings within the preschool classroom are discussed.