Various behavioral traits such as aggression, courtship, and locomotor activity are known to be determined by genetic factors in many organisms. Molecular machineries that buffer genetic variation of such behavioral traits would influence the evolvability of those traits. HSP90, one of the molecular chaperons, buffers genetic variation of morphological traits in fruit flies, and it may have similar effect on behavioral traits. In fact, a partial loss of HSP90 function impairs the strict link between molecular oscillations and behavioral activity and results in a large variation in individual behavioral activity patterns. This fact indicates that when HSP90 functions normally it buffers behavioral variation at within-individual level. If HSP90 buffers behavioral genetic variation, it would influence the evolvability of not only morphological traits, but also behavioral traits. To investigate this hypothesis, we examined the effect of HSP90 inhibition on the behavioral genetic variation in the circadian rhythms of locomotor activity in Drosophila melanogaster. We used 20 isogenic deficiency strains from DrosDel project to generate genetic variation. HSP90 function was inhibited by a pharmacological inhibitor, geldanamycin, to measure its effect on among-strains variation in locomotor activity.