Aristotle famously claimed that natural kinds form a single hierarchy of non-overlapping genera and species. This idea has influenced not only later Aristotelians, but also early modern biological taxonomists. However, many interpreters claim that the idea of a unique biological classifications plays no significant role in Aristotle’s own biological works. This would suggest either that Aristotle disregarded his metaphysical views when doing science, or that the underlying metaphysics of his biology is significantly different than traditionally assumed. I will argue that Aristotle has an explicit theory on the role of kinds in biology, that he thinks that the biological works are committed to unique classification into kinds (as traditionally assumed), but that biological explanation presupposes this classification only to a limited and precise extent.