This article addresses a child language stage that has figured prominently in the current debate on children's early linguistic competence: the Optional Infinitive (OI) stage, a relatively extended period during which children freely alternate between finite and nonfinite structures in contexts where adults only use finite forms. The study examines the assumption of previous research that children's utterances with finite or nonfinite forms are generated by productive morphological rules throughout the OI stage. Analyses of Dutch-speaking children's utterances indicate that at the onset of the OI stage finite verbs are infrequent and lexically restricted, and that lexical overlap between the verbs used in finite sentences and in children's nonfinite structures (root infinitives) grows. It is argued that these observations are not due to statistical properties of the corpus, the limited number of finite verbs, or a growing verb lexicon, but signal, instead, the morphological development of finiteness.