Classificatory behavior of 41 fourth graders enrolled in a science program at the National Zoological Park in Washington, D.C. was assessed on an instrument adapted from Piaget's interviews with children on class inclusion. Developmental levels were related to student performance in the program. Data analysis shows that students with a high understanding of class inclusion perform better in the program. The data also support Piaget's finding that a lag exists in children's application of class inclusion to animal classes. The results imply that educators keep in mind the developmental levels of students when planning programs and provide students with opportunities to extend emerging cognitive abilities to new situations. (Author/JN)