This study focuses on the semantic development of individual lexical items, as viewed from a semantic features perspective. It involves four narrow semantic domains, a sample of elementary school-children and their teachers, and two native language groups, English and Spanish. Semantic development is studied through the process of equivalence formation, the ability to group discriminably different things and to treat them as alike in some way. The results show significant developmental differences across age levels, with few differences between language groups. The salient features of the meanings of noun items are primarily Perceptible (size, shape, color, etc.) for younger subjects, but become increasingly and predominantly Nominal (a label used to name the group of words, e.g., food) among older subjects. The primary features of verb items remain predominantly Perceptible at all age levels. The results of this investigation differ significantly from two similar equivalence formation studies conducted among different populations. The meaning of a word and its subsequent development appear to be tied to an indivual's perception of the phenomenon represented by the lexical item and to his level of cognitive development. The results of this and other studies imply a broadening of the traditional concept of vocabulary development. (Author)