The acquisition of the passive in English poses a learnability problem. Most transitive verbs have passive forms (e.g., kick/was kicked by), tempting the child to form a productive rule of passivization deriving passive participles from active forms. However, some verbs cannot be passivized (e.g. cost/was cost by). Given that children do not receive negative evidence telling them which strings are ungrammatical, what prevents them from overgeneralizing a productive passive rule to the exceptional verbs (or if they do incorrectly passivize such verbs, how do they recover)? One possible solution is that children are conservative: they only generate passives for those verbs that they have heard in passive sentences in the input. We show that this proposal is incorrect: in children's spontaneous speech, they utter passive participles that they could not have heard in parental input, and in four experiments in which 3–8-year-olds were taught novel verbs in active sentences, they freely uttered passivized versions of them when describing new events. An alternative solution is that children at some point come to possess a semantic constraint distinguishing passivizable from nonpassivizable verbs. In two of the experiments, we show that children do not have an absolute constraint forbidding them to passivize nonactional verbs of perception or spatial relationships, although they passivize them somewhat more reluctantly than they do actional verbs. In two other experiments, we show that children's tendency to passivize depends on the mapping between thematic roles and grammatical functions specified by the verb: they selectively resist passivizing made-up verbs whose subjects are patients and whose objects are agents; and they are more likely to passivize spatial relation verbs with location subjects than with theme subjects. These trends are consistent with Jackendoff's “Thematic Hierarchy Condition” on the adult passive. However, we argue that the constraint on passive that adults obey, and that children approach, is somewhat different: passivizable verbs must have object arguments that are patients, either literally for action verbs, or in an extended abstract sense that individual languages can define for particular classes of nonactional verbs.