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The impact of wheat (Triticum aestivum L.) germplasm from the International Maize and Wheat Improvement Center (CIMMYT) in the highly productive environments of the developing world has been significant. However, 55% of the area sown to wheat in these countries is periodically affected by drought and the impact of CIMMYT germplasm on productivity in these areas is not clear. Our objective was to measure rates of yield improvement for the period 1979 through 1998 using yield data from CIMMYT’s Elite Spring Wheat Trial (ESWYT) and Semi-Arid Wheat Yield Trial (SAWYT). The mean yield of the five highest yielding entries from each site was expressed as a percent of the trial mean (%TM). The trial mean yield (TM) and the mean yield of locally adapted check cultivars (LC) were used to provide estimates of the productivity of each environment. Measuring rates of progress by means of %TM, TM, and LC were favored to the use of mean yield alone as variable annual rainfall and subsequent fluctuations in productivity influence yield in dry environments. Yearly rates of progress were determined by measuring change in %TM and change in TM. In environments yielding less than 4 Mg ha(-1) the respective increases in %TM and TM for SAWYT were 4.38 and 0.09% yr(-1). The equivalent rates for ESWYT were 0.34 and 0.19% yr(-1). In environments yielding 4 Mg ha(-1) or more the respective rates of progress in %TM and TM for SAWYT were 0.85 and 2.87% yr(-1) compared with 0.26 and 0.494% yr(-1) observed in the ESWYT. Comparisons of %TM, TM, and LC for representative locations in Mexico, Argentina, Syria, and Portugal demonstrated progress in %TM ranging from highly significant to stable, regardless of local fluctuations in productivity.