A sentence matching task is used to assess the processing effects of the ungrammaticality produced by violations of constraints on movement rules. Surprisingly, no effects of ungrammaticality were observed either for violations of the Specified Subject Constraint or the Subjacency constraint. These 'overgenerated' sentences could apparently be processed with the same fluency as fully grammatical controls, despite the fact that other types of ungrammaticality produced marked increases in matching times. It is proposed that the matching task utilizes a level of mental representation at which overgenerated sentences are indistinguishable from fully grammatical sentences. This implies a close correspondence between formal derivational mechanisms and features of the operation of the language processor.