The inclusion of social and emotional learning (SEL) curricula in preschools may help prevent emotional and behavioral problems. This study evaluated the effects of a SEL curriculum, "Strong Start Pre-K," on the social and emotional competence of 52 preschool students using a quasi-experimental, non-equivalent control group design. Teachers rated students' emotional regulation, internalizing behaviors, and the quality of the student-teacher relationship. Results indicated a significant decrease of internalizing behaviors and more improvement in the student-teacher relationship in the treatment conditions. Results also supported the use of the optional booster lessons contained in the curriculum. Treatment integrity and social validity ratings of "Strong Start Pre-K" were high. Limitations and implications of this study are addressed.