Iowa farmers produce soybeans profitably in fields infested with the soybean cyst nematode (SCN) by growing SCN-resistant soybean varieties that yield well and prevent large increases in SCN egg population densities. There are hundreds of SCN-resistant soybean varieties for Iowa. Almost all of the varieties contain SCN resistance genes from a single breeding line, called PI 88788. Because of widespread, repeated use of varieties with the same PI 88788 source of resistance, many SCN populations have developed increased reproduction on that type of resistance. It is no longer uncommon to see SCN females on roots of SCN-resistant soybean varieties in the Midwest. How does increased SCN reproduction on soybean varieties with the PI 88788 source of resistance affect soybean yields and the buildup of SCN egg population densities (numbers) in the soil? Results of Iowa State University field experiments reveal the answers.