To understand how children develop adult argument structure, we must understand the nature of syntactic and semantic representations during development. The present studies compare the performance of children aged 2;6 on the two intransitive alternations in English: patient ("Daddy is cooking the food"/"The food is cooking") and agent ("Daddy is cooking"). Children displayed abstract knowledge of both alternations, producing appropriate syntactic generalizations with novel verbs. These generalizations were adult-like in both "flexibility" and "constraint." Rather than limiting their generalizations to lexicalized frames, children produced sentences with a variety of nouns and pronouns. They also avoided semantic overgeneralizations, producing intransitive sentences that respected the event restrictions and animacy cues. Some generated semantically appropriate agent intransitives when discourse pressure favored patient intransitives, indicating a stronger command of the first alternation. This was in line with frequency distributions in child-directed speech. These findings suggest that children have early access to representations that permit flexible argument structure generalization.