The hyperkinetic mutants, Hk(1) and Hk(2), jump and fall over when an object moves near them. This behavior, the kinetogenic response, has been measured by the experimenter moving his hand above a vial containing a single fly and scoring the number of positive responses in fifty trials. The response is higher in Hk(1) than in Hk(2) and has remained so over a period of several years and in different genetic backgrounds. The Hk(1)/Hk(2) heterozygote also responds to movements, establishing the allelism of the two mutants.-When Sh(5), a shaker mutant at another locus on the X chromosome, is introduced into the same chromosome as Hk(1) or Hk(2), the response is reduced in proportion to the number of Sh(5) mutant genes added.-When Hk(1) or Hk(2) is heterozygous with a deficiency for the hyperkinetic region of the X chromosome, the expression is more abnormal than the respective Hk(1) or Hk(2) homozygote. This shows that the mutant genes are producing an altered gene product, or less of the normal, since one mutant gene by itself has a more abnormal expression than two. A tentative explanation has been offered for the observed mutant behaviors.