Data on the development of fundamental frequency patterns and the emergence of semantic relations during the "one word period" in child language development are reported in this study. The research focuses on the changes that occur as children progress from producing single words to sequences of single words and finally to producing two-word utterances. Four children were audio and video tape-recorded once every two weeks for 30 minutes until they began to produce primarily two-word utterances. All utterances were examined spectrographically to determine the characteristics of fundamental frequency and duration. The study presents data in tabular and graph form collected from one male subject, spanning the ages of 19 to 22 1/2 months, over seven sessions. Generally, the proportion of sequences and two-word utterances to single-word utterances increased over the observational sessions. The findings suggest that sequences of single words were planned as a linguistic unit, since they share fundamental frequency characteristics with unified two-word utterances, despite the fact that a pause had intervened between two words during execution. It is concluded that sequences of single words appear to be structured units that reflect the transition between single words and multiple-word utterances. (EJS)