Tightly linked gene markers at the white locus of Drosophila were placed, by repeated backcrossing, on each of two genetic backgrounds. Ten genotypes representing various combinations of homo- and heterozygosity were studied in a Hirsch-type geotactic maze. In general, the effects of genetic background, dominance, and epistasis were found to exceed the additive effects of gene substitutions. The results domonstrate the feasibility of analyzing the contributions of single genes to behaviors known to be polygenically determined.