This paper investigates the conditioning role of R&D employee training in the effect of the stock of technological knowledge on firm R&D productivity. We suggest that the development of R&D human capital through training enhances firm-specific technological competence, thereby enabling firms to better utilize their stock of technological knowledge as a source of technological opportunities. Specifically, while a trap of path dependency may hamper the proper utilization of the firm-specific stock of technological knowledge, R&D employee training reinforces the standing-on-the-shoulders effect of the stock of technological knowledge, thereby offsetting, at least partially, its fishing-out effect due to depleting technological opportunities. Using panel data of Korean manufacturing firms, we show that R&D employee training positively moderates the effect of the stock of technological knowledge on firm R&D productivity. Furthermore, we suggest several factors that influence the positive moderating effect of R&D employee training such as the degree of knowledge sharing among R&D employees within each firm, the strength of industry R&D appropriability, and the industry R&D intensity.