Two-year-olds use the sentence structures verbs appear in—subcategorization frames—to guide verb learning. This is syntactic bootstrapping. This study probed the developmental origins of this ability. The structure-mapping account proposes that children begin with a bias toward one-to-one mapping between nouns in sentences and participant roles in events. This account predicts that subcategorization frames should guide very early verb learning, if the number of nouns in the sentences is informative. In 3 experiments, one hundred and thirty-six 21- and 19-month-olds assigned appropriately different interpretations to novel verbs in transitive (“He’s gorping him!”) versus intransitive sentences (“He’s gorping!”) differing in their number of nouns. Thus, subcategorization frames guide verb interpretation in very young children. These findings constrain theoretical proposals about mechanisms for syntactic bootstrapping.