This study examines the relationship between on-the-job training and job performance among 3,408 telephone operators in a large unionized telecommunications company. We utilize individual data on monthly training hours and job performance over a five-month period as provided by the company’s electronic monitoring system. Results indicate that the receipt of on-the-job training is associated with significantly higher productivity over time, when unobserved individual heterogeneity is taken into account. Moreover, workers with lower pre-training proficiency show greater improvements over time than those with higher pre-training proficiency. Finally, whether the training is provided by a supervisor or a peer also matters. Workers with lower proficiency achieve greater productivity gains through supervisor training, while workers with higher proficiency achieve greater productivity gains through peer training.