A study involving eight boys with autism and eight boys with Down syndrome (ages 6-10) with similar adaptive behavior levels used an ecological approach to study the in-depth structure of the behavior stream. Narrative records were collected and analyzed based on duration and frequency of behaviors. Each record was analyzed by marking the behavior stream into activity units (AUs), which are naturally occurring chunks of behavior from the perspective of the child that proceed in a constant psychological direction. Findings revealed that children with autism exhibited AUs of shorter duration and with less overlap. No differences in features related to the setting or in qualitative aspects of AUs were found. Children with autism were more likely to use physical mechanisms; children with Down syndrome used more gestures. Analysis of the duration of intention of AUs revealed that children spent similar amounts of time participating in all but two types of activities. Children with autism spent more time readying their environment and children with Down syndrome spent more time eating and drinking. Subcategory analysis revealed that children with autism were four times more likely to fail to respond. (Contains 165 references.) (Author/CR)