Employing a cross-age design, this study examined students' alternative conceptions in animal classification at the elementary, secondary, and college levels. Based on a previous study that made use of clinical interviews and a classification task, subjects (N = 468) were administered a multiple-choice/free-response instrument that probed understanding of the concept animal, the vertebrate/invertebrate distinction, and the principal vertebrate classes. Results suggest that students subscribe to a highly restricted view of animals; applying the label almost exclusively to vertebrates, especially to common mammals. When asked to distinguish between vertebrate and invertebrate animals and to classify several species into vertebrate groups, a wide range of alternative conceptions emerged. Cross-age comparisons indicate that many of these alternative views remain intact throughout the school years, while others yield more readily to formal instruction and/or nonschool experiences. Considered within the context of a neoconstructivist view of learning, several suggestions are offered for teaching concepts in animal classification.