In most extant theories of form-class acquisition, the relationship between the semantic and formal properties of each class is held to be similar. This paper presents arguments that this relationship is in fact different for nouns than for any other lexical class. The semantic notion of concrete object reference functions both as an entree into the category and subsequently as the core which holds this category together, as its subclasses admit of no unified structural description. The basic categorial split in the world's languages is between ' ‘noun’ and ' ‘other'. Children cannot initially assume that the notions ' ‘action’ ' ‘stative property’ ' ‘locative relation’ will provide entries into the categories verb, adjective, preposition, respectively, first because these categorial distinctions are not universally attested, and second because in those languages that do have them, there is a much looser fit between notion and category. For verbs, for example, small-scale combinatorial properties provide a more reliable characterization than ' ‘action'. It is further argued that a semantic-structural characterization of noun as ' ‘head of argument phrase’ provides the child with a less effective entry into the category: not all arguments are noun-headed, and conversely, nouns head constructions which do not function as arguments. In sum, the relationship between the category noun and its semantic characterization is special. The paper concludes with a critique of a previous model ofform-class acquisition (Maratsos and Chalkley 1980) in the light of the arguments presented here.