Ethnobiologists debate whether folk biological classifiers are natural historians attending primarily to the morphology of organisms or are pragmatists concerned primarily with utility. We argue that this question is best understood as a problem in intracultural variation: the relative importance of form and function depends on who is asked to judge the similarity of organisms as well as how they are asked to judge it. We find that expert fishermen judge similarities among fish on both functional and morphological criteria, while novices judge on morphological criteria alone and thereby approach the scientific classification of fish more closely than experts. Experts also vary more than do novices, presumably because they control more different kinds of knowledge on which to base a similarity judgment.