The present work investigated whether emotion regulation and social preference were associated with participant roles in bullying as a function of the quality of the relationship with teachers. Participants were 332 children (172 boys), in the age of 42-76 months (M = 58.74; SD = 7.84). Peer nominations were employed to assess social preference and participant roles (bullying, victimization, defending the victim, and outsider behavior). Teachers completed the Emotion Regulation Checklist, which yields the dimensions of emotion regulation and lability/negativity, and the Student-Teacher Relationships Scale, to evaluate conflict and closeness with the teacher. Multilevel models highlighted that emotional lability was positively associated with bullying and outsider behavior, emotion regulation was positively related to bullying and defending behavior, and social preference was negatively associated with bullying and victimization and positively with defending behavior. Interactions indicated that lability and low social preference were associated with bullying, and emotion regulation with outsider behavior, in children with a conflictual relationship with the teacher whereas social preference was related to defending behavior in children with a close relationship with the teacher. Results are discussed highlighting the importance of the quality of teacher-child relationship and the relevance of intervention programs aimed at promoting social wellbeing in preschool. (author abstract)