Using a Hirsch classification maze, selection was made for positive and negative geotactic behaviors in three different strains of Drosophila pseudoobscura. Hybridization studies were then carried out with flies from the diverged strains. The geotactic scores of the parents and F1 flies indicate that both negative and positive geotactic behaviors in these strains are strongly influenced by genes in the X chromosome. Additional hybridization studies using flies from strains with similar phenotypes suggest that the diverged strains contain similar alleles and the number of loci in the X chromosomes responsible for these types of behavior is limited. The loci may be highly organized in the X chromosome.