Firms that open up their organizational boundaries and access valuable external sources of knowledge can create new opportunities for innovation. However, little is known about this conjecture in the business-to-business (B-to-B) service context: whether B-to-B service firms utilize external knowledge and ideas for innovation, and if so, which types of partners they collaborate with in their innovation activities. This research is a step toward filling this void. Tobit regression analysis on secondary data was performed using various types of external partners as sources of innovation to determine the innovation performance of transportation and logistics service firms. The results show that customers, suppliers, and competitors (in descending order) contribute to service improvement, and customers contribute to the development of services that are new to the firm. In contrast, the use of universities and consultants as sources of innovation does not seem to immediately affect innovation performance in the transportation and logistics service industry.