The present study investigated the role of the causal status of features and feature type in biological categorizations by young children. Study 1 showed that 5-year-olds are more strongly influenced by causal features than effect features; 4-year-olds exhibit no such tendency. There therefore appears to be a conceptual change between the ages of 4 and 5 in the evaluation of the causal relations between features that characterize biological categories. The aim of Study 2 was to identify the nature of the abstract beliefs that underlie children's categorial choices. Results show that 5-year-olds base category choices on causal features only when the status of the cause is associated with an internal feature and not if the feature is merely a surface feature. Children thus use biological knowledge to perform the task. (Contains 3 figures and 1 table.)