Analyzing data obtained from interviews at 653 Spanish industrial plants in 1997, the authors investigate factors affecting the choice between two ways of filling supervisory and skilled technician positions: internal promotion of blue-collar workers, and external recruitment. The use of internal promotion was positively correlated with efforts by plants to measure employees' skills, as well as with the specificity of blue-collar workers' human capital investment at the plant. Contrary to expectations, no evidence is found that the use or efficiency of other incentive systems, such as variable pay, had a significant influence on the degree to which internal promotion was used. The authors interpret their results as preliminary evidence that internal promotions are used to protect and favor specific investments, especially those made by firms to assess their workers' skills. © by Cornell University.