The dynamics of gametic frequency change have been analyzed in two experimental plant populations. Individual plants were scored for their genotype at four enzyme loci, and four-locus joint gametic frequencies were estimated from the genotypic data over generations. Striking correlations developed in allelic state over loci, including correlations between nonlinked loci, as these populations evolved. Furthermore, the same pair of four-locus complementary gametic types came into marked excess in the late generations of both populations. The results demonstrate that natural selection acted to structure the genetic resources of these populations into sets of highly interacting, coadapted gene complexes. They also provide evidence, at the level of the gene, that selection operating on correlated multilocus units is an important determinant of population structure.