Researchers have found it difficult to measure various forms of indirect aggression, such as exclusion from groups, because such behaviors are difficult to observe in field settings such as school playgrounds. This study examined gender and developmental differences in aggression, investigated across-gender aggression, and looked at teachers' estimates of students' aggression. An aggression scale was administered to students across four grades in two Catholic high schools and four Catholic primary schools. Subjects then estimated on a five point scale how often they saw the listed aggressive behaviors among students in their own grade. Results confirmed earlier findings that there are gender and developmental differences in aggression. Boys used more physical aggression strategies than girls and older girls used more indirect forms of aggression than did boys. Boys tended to have higher levels of verbal aggression than girls. Across genders, boys estimated that they used less aggression toward girls but girls disagreed. Girls used more physical aggression against boys in only one grade level and girls always used less indirect aggression. Teachers estimated that both boys and girls had higher levels of aggression than levels estimated by the students. Teachers did not know as much about indirect within-gender aggression as did students. (RJM)