Objective: The vast majority of children with an autism spectrum disorder (ASD) attend public preschools at some point in their childhood. Community preschool practices often are not evidence based, and almost none target the prelinguistic core deficits of ASD. This study investigated the effectiveness of public preschool teachers implementing a validated intervention (the Joint Attention and Symbolic Play/Engagement and Regulation intervention; JASP/ER) on a core deficit of autism, initiating joint attention. Method: Sixteen dyads (preschoolers with ASD and the public school teachers who worked in the child's classroom) were randomly assigned to the 6-week JASP/ER intervention or a control group. Results: At the end of the intervention, JASP/ER teachers used more JASP/ER strategies than the control teachers, and JASP/ER preschoolers used more joint attention in their classroom than control children. Additionally, JASP/ER children spent more time in supported engagement and less time in object engagement than control preschoolers on a taped play interaction. Conclusions: Findings suggest that teachers were able to improve a core deficit of children with ASD in a public preschool context. (Contains 1 figure and 7 tables.)