With the increased presence in the Corn Belt of combine-mounted yield monitors generating yield maps, more producers are asking questions like 'when do spatial yield differences become important?' and 'should I change my production practices because of observed yield differences?' This study examined variations in corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] yields over time. Yield were monitored from four continuous corn plots, four continuous soybean plots, and eight corn-soybean rotation plots over a 10-yr period from 1986 through 1995 at two Minnesota locations and one Wisconsin location. At each location, all plots were on a similar soil type in a uniform 2-acre field. Plot size was at least 450 sq ft. At each location, each of the four continuous corn plots and each of the four continuous soybean lots produced the highest yield compared with the other three plots at least once during the 10 yr. In any single year for both continuous corn and continuous soybean, a yield range among the four plots at each location of more than 25% occurred in one-fourth of the growing seasons studied. When averaged over 10 yr, however, this yield range among the four continuous corn plots and among the four continuous soybean plots at each location was less than 10%, and in five of six cases yield differences among the four plots were not significantly different. At each location, the yield range averaged at least 20 bu/acre for corn and 6 bu/acre for soybean. Yield variability among years was approximately three times greater for soybean and four times greater for corn than was variability among plots. Similar results were observed for the plots in the corn-soybean rotation. These results demonstrate a relatively high amount of inherent yield variability, and suggest producers should not change management practices to influence yields when small yield differences occur (areas yielding up to 20 to 25% less than the field average), unless the differences are shown to be consistent over years.