OBJECTIVES: To compare the effects of two 9-month parent-implemented interventions within the Early Social Interaction (ESI) Project. Both individual-ESI, offered 2 or 3 times per week at home or in the community, and group-ESI, offered once per week in a clinic, taught parents how to embed strategies to support social communication throughout everyday activities. METHODS: Participants in the randomized controlled trial included 82 children diagnosed with autism spectrum disorder at 16 to 20 months. Children were matched on pretreatment nonverbal developmental level and pairs were randomly assigned to treatment condition. Child outcomes included measures of social communication, autism symptoms, adaptive behavior, and developmental level. Child outcomes are reported from baseline to the end of the 9-month interventions. RESULTS: Children in individual-ESI showed differential change on a standardized examiner-administered observational measure of social communication, as they improved at a faster rate than children in group-ESI. Individual-ESI also showed differential efficacy on a parent report measure of communication, daily living, and social skills, as they showed improvement or stability, whereas group-ESI led to worsening or no significant change on these skills. Finally, individual-ESI showed differential change on examiner-administered measures of receptive language skills, as children in individual-ESI improved significantly, whereas group-ESI showed no change. CONCLUSIONS: These findings support the efficacy of individual-ESI compared with group-ESI on child outcomes, suggesting the importance of individualized parent coaching in natural environments. The efficacy of a parent-implemented intervention using little professional time has potential for community viability, which is particularly important in light of the lack of main effects on child outcomes of most other parent-implemented interventions.