One of the problems that has to be dealt with by theorists of early language acquisition theory is the mismatch between semantic constructs, like Agent, and syntactic ones, like subject. It is proposed that the linguistic system is based on semantic features that are more fine-grained than thematic roles, and that selection of subject and direct object can be accounted for by merely four semantic features. These features are conceived of as properties of participants in the lexical entries of verbs, and in this respect, too, they are unlike thematic roles, which are ascribed to NPs in sentences. Thematic roles play a part only in the realization of certain other arguments, notably, the oblique object. It is shown that this different treatment of direct and oblique objects permits a parsimonious explanation of certain linguistic regularities that have posed problems for other theories. Early language acquisition can be explained in terms of the acquisition of these semantic features, and this account thus supersedes the semantic assimilation hypothesis proposed previously to deal with the lack of congruence between thematic roles and syntactic categories.