The hyperkinetic mutants, Hk¹ and Hk², 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 Hk¹ than in Hk² and has remained so over a period of several years and in different genetic backgrounds. The Hk¹/Hk² heterozygote also responds to movements, establishing the allelism of the two mutants.—When Sh⁵, a shaker mutant at another locus on the X chromosome, is introduced into the same chromosome as Hk¹ or Hk², the response is reduced in proportion to the number of Sh⁵ mutant genes added.—When Hk¹ or Hk² is heterozygous with a deficiency for the hyperkinetic region of the X chromosome, the expression is more abnormal than the respective Hk¹ or Hk² 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.