The study investigated the effects of structured and unstructured cooperating groups on students' behaviors, discourse and learning in junior high school. One hundred and sixty-four grade 9 students participated in the study. The students were videotaped as they worked in three to four person, mixed-gender and ability groups on a science-based categorization activity. The results show that the students in structured cooperating groups demonstrated more cooperative and helping behaviors such as giving more elaborated help and guided directions to assist understanding than their peers in the unstructured groups. Moreover, they demonstrated more complex thinking and problem-solving skills both in their discourse and their responses on the follow-up learning probe. These findings are discussed in the context of the importance of structuring cooperative learning experiences if students are to attain the benefits widely attributed to this approach to learning. (Contains 3 tables.)