One way to support the social engagement of children with autism spectrum disorders (ASD) is by identifying features of natural environments that increase the likelihood of social interaction. This descriptive study was an examination of the (a) social engagement of 68 preschoolers with ASD in classrooms, (b) ecological features of classrooms that promoted social engagement, and (c) relationships between social engagement and ecological features using the Code for Active Student Participation and Engagement-Revised, an ecobehavioral observational system. Overall, children without ASD were most likely to be socially engaged with peers in the Books and Food/Snack classroom areas, when participating in book or large motor behaviors, in small groups with peers or large groups with an adult, and during child-initiated activities. Implications for practice and future research are highlighted. (Contains 4 figures and 5 tables.)