This study explored and documented Taiwanese students' alternative conceptions of animal classification. We examined the understanding of the "animal", "vertebrate" and "invertebrate", "fish", "amphibian", "reptile", "bird", and "mammal" concepts among elementary, junior high school and senior high school, and university students in a sample population of close to 2,000 students. Using clinical interviews, sorting tasks, and a two-tiered diagnostic instrument, we documented the frequencies of a wide range of ideas and compared our findings with those of studies in the United Kingdom, New Zealand, and the United States. The most important results indicate that for most students, the concept label "animal" refers to vertebrates, especially to common, well-known mammals and birds; the most common attributes used by students to define animals are movement and viability; many students had difficulty in making the distinction between vertebrates and invertebrates, and between reptiles and amphibians; and students tended to use external morphology, habitat, and movement in distinguishing between common, well-known vertebrates and invertebrates, and certain vertebrate animals pose special conceptual problems for students by virtue of their external morphology and habitat (e.g., the penguin and octopus). We contend that insights gained from this study will provide useful suggestions for Taiwanese science curriculum designers, science teachers, and researchers involved in the new 9-year curriculum reform. (Contains 5 tables and 5 figures.)