Identifying alternative conceptions is a vital part of teaching and learning in science because it provides information about students' ideas to both themselves and their teachers. In this context, the purpose of the current study is twofold. The first aim is to examine high school students' alternative conceptions about general characteristics and classification of invertebrates. The second aim is to elicit high school students' ability to classify invertebrates. A total of 127 grade 10 and 129 grade 11 students (aged 15-17 years) from four Turkish Anatolian high schools participated in the study. Data were collected using the Animal Classification Test (ACT) and through interviews. A wide range of alternative conceptions emerged and the origin of these alternative conceptions are discussed. Furthermore, the results indicated that although the students could correctly recognize the difference between vertebrates and invertebrates (Phyla), they had failed to recognize the corresponding sub-categories (Classes). As a consequence, it was asserted that when the students classified the animals, they took into consideration only the animals' external views, habitats, nutrition and movement types and similarities in the functions of their organs. Thus the students used analogical approaches based on empirical classification criteria.