The present study provides a person-centered view on teachers' adherence to the KiVa antibullying curriculum over a school year. Factor mixture modeling was used to examine how teachers (N = 282) differed in their implementation profiles and multinomial logistic regression was used to identify factors related to these profiles. On the basis of lesson adherence across time, 3 types of implementers emerged: (a) high implementers (53%) started at a very high level and remained so over time, (b) moderate implementers (30%) consistently utilized more than half of the lesson material, and (c) surrenders (17%) started at a high level that soon declined. Teachers' beliefs toward program effectiveness were positively associated with starting at higher levels of fidelity (high and surrenders), whereas principal support for antibullying work predicted maintaining the initial level of implementation over the school year (high and moderate). Finally, good lesson preparation and student engagement were associated with a higher levels of implementation throughout the school year (the high group). Neither participation in preimplementation training nor classroom management skills were related to 3 implementation profiles. The findings highlight the importance of individual and interpersonal factors for successful implementation of school-based bullying prevention programs.