The influence of the child's knowledge base, in terms of event schemas, on symbolic play behavior was investigated. The pretend play behavior of 10 mother-child (2-0 to 2-4) dyads was observed in 2 play contexts. Play was examined for thematic content and the following structural components: self-other relations, substitute/imaginary objects, action integration, and planfulness. The highest levels of symbolic play behavior emerged in pretense episodes whose thematic content was event based. Additionally, thematic content affected the respective roles of mother and child in the construction of pretense. In pretense activity based on themes with which the child was familiar (e.g., routine events), the child, as well as the mother, participated in advanced levels of symbolic play activity, coconstructing pretense. In pretense based on themes unfamiliar to the child, the mother was almost exclusively responsible for the pretense. Thus, the development of child symbolic play appears to be related to the knowledge base in that its emergence is domain-specific--limited to themes for which the child has knowledge--before being more widely manifested.