There has been increasing interest in the promotion of social and emotional learning in schools, and research has shown positive outcomes. However, relatively few studies have been conducted in kindergarten classrooms or considered the feasibility of kindergarten implementation. This study examined the effects of Strong Start on the social and emotional competence of 67 kindergarten students, using a time-series design. Four kindergarten teachers taught the ten Strong Start lessons in their classrooms. Results indicated gains in students' prosocial behaviors and decreases in internalizing behaviors, as rated by teachers and parents. Implementation integrity and teachers' ratings of social validity were high, suggesting the program's feasibility and potential effectiveness in natural classroom settings. Limitations and implications are discussed.