The release of the corn (Zea mays L.) synthetic, BS9(CB)C4, to the hybrid seed industry in 1982 was a significant event to host-plant resistance investigations because it was the first released Corn Belt synthetic specifically developed and selected for resistance to European corn borer (ECB), Ostrinia nubilalis (Hubner), for the whole life of the plant. To determine the efficacy of S(,1) recurrent selection for resistance to the two generations of ECB normally found in Iowa, the base population and four succeeding cycles of selection of BS9 were evaluated for ECB resistance and correlated effects on agronomic traits;Significant increases were found from BS9C0 to BS9(CB)C4 for resistance to first generation (leaf-feeding), second generation (sheath-collar feeding), and stalk tunneling (cavity counts). Averaged over means of S(,1) lines, populations per se, and population testcrosses, ECB damage ratings based on a 1 to 9 scale decreased from 3.9 to 3.0 for first generation and from 6.0 to 4.3 for second generation in BS9C0 and BS9(CB)C4, respectively. Similarly, the average of populations per se and population testcrosses for cavity counts (one cavity is ca. 2.5 cm) decreased from 8.2 to 4.0 in the two populations, respectively;The increase in resistance in populations of BS9 reduced yield losses under artificial infestations of ECB, but the reduction was not sufficient to compensate for the loss in yielding ability that occurred as a correlated effect from selection for ECB resistance. Reduction in the grain yield from BS9C0 to BS9(CB)C4 under no artificial infestation was estimated to be 8.4% caused by changes in gene frequency due to selection and 18.8% caused by inbreeding depression due to drift. Most of the yield reduction, therefore, was caused by a random fixation of heterozygous loci, which may have been increased because of linkages to alleles of other traits under direct and indirect selection. Ear diameter, ear height, and plant height decreased in BS9 possibly as a result of indirect selection for shorter internode length to improve stalk hardness and, thereby, stalk-tunneling resistance;S(,1) recurrent selection, therefore, was effective in increasing resistance throughout the life of the corn plant, but unfavorable responses in other agronomic traits, particularly in grain yield, suggest that the selection criteria for ECB resistance should include yield.