Theoretical correspondences between the language and play development of young children and developmental aspects of play and speech are discussed prior to a report of a study comparing categories of cognitive complexity nested within social categories of solitary play and self speech. Participants were 36 preschool boys and girls approximately 36, 48, and 60 months of age. Data were collected in six 5-minute observations for each child. Each play episode observed was coded for social participation complexity and utterances were coded for self speech, collective monologue, and socially adapted speech. Solitary play episodes and self speech utterances were then coded on dimensions of cognitive complexity. Results substantiate the view that categories of social play and social speech do not form hierarchical levels of development. The frequency of solitary play decreased significantly with age. A significant increase across age groups was found for group play. Interestingly, parallel play increased from age 3 to 4 and decreased from 4 to 5 years, suggesting that parallel play may be a formative context for the child's growing discrimination between private and social aspects of psychological functioning. Both solitary play and self speech showed developmental changes for cognitive categories. Implications of the findings for development and educational practices are discussed. (RH)