The development of specified tense and number morphology in child Catalan and Spanish is found to correlate with the onset of overt subject use. The data come from four monolingual child Catalan-speakers (from the Serrà & Solé corpus) and one monolingual child Spanish-speaker (from the Linaza corpus) who were studied longitudinally from 1:0 to 3; 6, approximately. The simultaneous emergence of tense and number morphology on one hand and overt subjects on the other in the children's speech is taken as evidence that a particular aspect of Universal Grammar, Case Theory, determines the possible co-occurrences of verbal inflections and subject types in developing grammatical systems. Parallels in verbal inflectional development are found in other child languages, while such parallels are not found in regard to subject use. Possible modifications to Case Theory, which would allow a unified account of the cross-linguistic developmental patterns of subject use, are considered. The possibility of explaining the early absence of overt subjects in these null subject languages as the result of an early sentence processing deficit is explored and rejected. The children's knowledge of whether their language is a null subject or overt subject language even before acquiring adult-like verbal inflection is taken as further evidence for what has been called ‘early convergence’ on parameter settings.