The purpose of this study was to determine the effectiveness of in-service training on teachers' knowledge of effective classroom management strategies. Twenty teachers from Jos metropolis participated. A single group pre-post test design was used, with a one-day training on classroom management techniques as the independent variable and the number of strategies that teachers suggest as effective for managing student behavior as the dependent variable. An open-ended structured interview was used to measure the dependent variable. The study found that the number of strategies that teachers believe are effective for classroom management was significantly higher after the training, indicating that the training was effective in improving teachers' knowledge about classroom management. Specifically, the number of proactive strategies that teachers believed were effective for classroom management significantly increased after training, whereas the number of reactive strategies remained unchanged. Thus, teachers' knowledge about effective classroom management strategies can be effectively modified by a brief, one-day in-service training.