Soybean cyst nematode, Heterodera glycines, is a microscopic, unsegmented plant-parasitic roundworm that attacks the roots of soybeans. Many plant-parasitic nematodes are believed to be native to the United States, but soybean cyst nematode likely was introduced into the United States from Japan. Soybean cyst nematode was first discovered in the United States in 1954 in North Carolina (Winstead et al., 1955) and since has spread to 26 additional states in the Southeast and Midwest (Figure 1) (Noel, 1992). The nematode was first discovered in Iowa in Winnebago County in 1978. More than 70% of approximately 400 randomly selected Iowa fields sampled in 1995 and 1996 were infested with the nematode (Workneh et al., 1998). This nematode is a widespread and serious threat to profitable soybean production throughout the Midwest.