Genetic analysis of differences in the activity of DFP-sensitive juvenile hormone esterase in 24-hour-old D. virilis flies was performed. Lines of D. virilis resistant and sensitive to long-term heat stress were analyzed under normal conditions. The level of juvenile hormone hydrolysis by this esterase was higher in the resistant than in the sensitive line. Interlinear differences in the enzyme activity were shown to be controlled monogenically or by a block of tightly linked genes. The allele of the resistant line was dominant.