Language acquisition is frequently characterized as a process where learning proceeds implicitly, i.e. incidentally and in absence of awareness of what was learned. This article reports the results of an experiment that investigated whether adults can acquire the syntactic structure of a novel language implicitly. Experimental subjects were trained on a semi-artificial grammar under incidental learning conditions, and then tested to determine whether learning took place and to assess whether learning resulted in unconscious knowledge. The results indicate that adults are able to acquire syntactic knowledge of a new language under incidental learning conditions, while processing sentences for meaning, without the benefit of corrective feedback and after a relatively brief exposure period. The results also show that learners are able to transfer knowledge to stimuli with the same underlying structure but new surface features. The measures of awareness further suggest that subjects were aware of having acquired knowledge, but that they were unaware of the nature of this knowledge. The experiment thus provides evidence for the implicit learning of natural language syntax.