The underlying semantic structure of the complete lexicon of four Hebrew-speaking infants at the one-word stage of lexical development was examined. Particular attention was given to the timing of word acquisition and the patterns of identified relationships among words. Three types of interrelationships among early words were explored: (1) relations between equivalent members of the same category; (2) relations between contrast and synonymity; and (3) relations between superordinate-subordinate terms. Data were recorded in diary form by infants' mothers. In an end-state analysis, the complete lexicon of each child during the last week of study was partitioned into semantic fields of adult language. Subsequently, in a developmental analysis, the course of generation of each semantic class was traced and changes over time in the underlying structure of each category were evaluated. Findings suggested that the one-word stage is divided into two developmental phases. During an initial phase mainly unrelated words are learned. Lexical acquisition is characterized by a horizontal covering of as many categories as possible, each containing only a few members. In the second phase, an abrupt change in the rate of learning new words is evident. This phase is characterized by a vertical differentiation within existing categories and by much faster and more efficient acquisition. During the second phase, the child lexicalizes much finer distinctions than before. (RH)