Tensile tests were performed on specimens in quenched and tempered and thermally aged conditions over a wide temperature range (300-873 K) to assess the occurrence of serrated flow, a manifestation of dynamic strain aging (DSA), in 9Cr-1Mo ferritic steel, with an emphasis on the influence of prior thermal aging on serrated yielding. The alloy exhibited jerky/serrated flow in the load-elongation curves at intermediate temperatures. Types A, B, and C serrations were observed, depending on the test temperature and applied strain rate. The apparent activation energy of 83 kJ mol-1 measured for serrated flow suggests that diffusion of an interstitial solute such as carbon is responsible for dynamic strain aging in 9Cr-1Mo steel. Prior thermal aging at 793 K for 5000 h and at 873 K for 1000 and 5000 h resulted in a significant decrease in the height of serrations, i.e. the magnitude of the stress drop, as well as an increase in the critical strain for the onset of serrations. Both of these observations indicate reduced propensity to DSA as a result of increased precipitate sinks as well as a reduced carbon concentration in solid solution owing to an increased density of carbides in the thermally aged conditions. Reduced propensity to DSA resulted in a significant reduction in the strength values at intermediate temperatures. © 1999 IoM Communications Ltd.