Selected spontaneous errors of word choice made between the ages of about 2 and 5 by 2 children whose language development has been followed longitudinally were analyzed for clues to semantic development. The errors involved the children's occasional replacement of a contextually required word by a semantically similar word after weeks or months of using both words appropriately. Because the errors were not present from the beginning and because correct usage prevailed most of the time, the errors cannot be explained by existing accounts of semantic development, which ascribe children's word-choice errors to initial linguistic immaturity. A plausible alternative account likens the errors to adult "slips of the tongue" in which the speaker, in the process of constructing a sentence to express a given meaning, chooses incorrectly among competing semantically related words. Interpreted in this way, the errors indicate that the process of drawing words into structured semantic systems based on shared meaning components begins much earlier than experimental studies have suggested. They also provide evidence for certain differences between children and adults in the planning and monitoring of speech.