Peroxidase is an enzyme present in soybean [Glycine max (L.) Merrill] seed coats and is characterized as either high (dominant allele) or low (recessive allele) activity. Cultivar Cutler 71 is a mixture of high and low seed coat peroxidase genotypes. Mechanical mixtures of 1 high: 1 low peroxidase seed coat genotypes were made and planted near West Lafayette, IN and Ames, IA starting in 1977. Our objective was to follow the gene frequency for peroxidase at two locations for a number of years. Starting in 1977 and continuing to the present, plots were harvested at the two locations and two 1000-seed samples were assayed for seed coat peroxidase each year. The 1:1 ratio fluctuated slightly from 1977 to 1982 at West Lafayette and from 1977 to 1988 at Iowa. A linear trend favoring low seed coat peroxidase was observed at both locations since 1982 and 1988. For example, in 2006 at West Lafayette the ratio was 0.21 high:1.79 low peroxidase and in 2006 at Ames, the ratio was 0.33 high:1.67 low peroxidase. Agronomic tests were conducted with high and low genotypes selected from specific years from both locations. Statistically significant yield differences were noticed but were not associated with seed coat peroxidase genotype or locations. Additional tests will be done in an attempt to explain the preference for low seed coat peroxidase or, conversely, to explain the selection against high seed coat peroxidase activity.