We investigated the implicit learning of a linguistically relevant variable (animacy) in a natural language context (namely, the relation of forms of determiners to semantics). Trial by trial subjective measures indicated that exposure to a form-animacy regularity led to unconscious knowledge of that regularity. Under the same conditions, people did not learn about another form-meaning regularity when a linguistically arbitrary variable was used instead of animacy (size relative to a dog). Implicit learning is constrained to acquire unconscious knowledge about features with high prior probabilities of being relevant in that domain.