Inbred recycling has narrowed the diversity of U.S. Corn Belt germplasm. Exotic populations, landraces, open-pollinated populations, and synthetic populations are possible sources of new germplasm. The objectives of this study were to (i) identify the best maize (Zea mays L.) populations among a diverse group of 17 early populations for improving an elite single cross, LH 227 x LH 295, and (ii) determine the genetic diversity among the populations and Minnesota inbreds from the Iowa Stiff Stalk Synthetic (BSSS) and non-BSSS heterotic groups. The LH 227 x LH 295 single cross, the two parental inbreds, and the testcrosses of the 17 populations to LH 227 and to LH 295 were evaluated at three Minnesota locations in 2002. The genetic diversity among the populations and Minnesota inbreds A632 and A679 (BSSS) and A619 and A682 (non-BSSS) was assessed with 55 simple sequence repeat (SSR) markers. Yield data were used in a quantitative genetic model to estimate, in each population, the relative frequency of favorable dominant alleles (lp̄lμ) not found in LH 227 x LH 295. Cateto, AS5, ASA(FNI)C5, and ASA(FI)C5 had the highest lp̄lμ values for yield. Cluster analysis of SSR data grouped these four populations in one cluster and the Minnesota inbreds in a different cluster. We concluded that Cateto, a synthetic from South America adapted to the northern Corn Belt, is a suitable population for improving LH 227 x LH 295.