This study examines the relationship between informal training and job performance among 2,803 telephone operators in a large unionized U.S. telecommunications company. The authors analyze individual-level data on monthly training hours and job performance over a five-month period in 2001 as provided by the company's electronic monitoring system. The results indicate that the receipt of informal training was associated with higher productivity over time, when unobserved individual heterogeneity is taken into account. Workers with lower pre-training proficiency showed greater improvements over time than did those with higher pre-training proficiency. Finally, whether the trainer was a supervisor or a peer also mattered: workers with below-average pre-training proficiency achieved greater productivity gains through supervisor training, while workers with average pre-training proficiency achieved greater productivity gains through peer training.