1. Transgenic Manipulation of the Gene Copy Number of Hsp70, Which Encodes the Major Inducible Heat-Shock Protein of Drosophila Melanogaster (Hsp70), Affects Both Hsp70 Levels and Inducible Thermotolerance in the Laboratory; Here Parallel Effects in Transgenic Drosophila Larvae Undergoing Natural or Simulated Natural Thermal Stress Are Demonstrated. 2. Necrotic Fruit Was Infested with Larvae of Either of Two Transgenic Strains, One Transformed with 12 Extra Copies of the Hsp70 Gene (Extra-Copy Strain) and a Sister Strain Possessing Only the Wild-Type Number (10) of Hsp70 Genes (Excision Strain), and Then Allowed to Heat to Variable Extents. 3. as the Intensity of Thermal Stress Increased, the Consequences of Extra Hsp70 Copies Reversed. after No or Moderate Thermal Stress, Excision Larvae Survived Better Than Did Extra Copy Larvae. by Contrast, Extra Copy Larvae Tolerated Intense Hyperthermia Better Than Did Excision Larvae. 4. These Results Establish that the Hsp70-Mediated Enhancement of Stress Tolerance, Previously Demonstrated Only for Artificial Stress Regimes in the Laboratory, Extends to Natural Stress Regimes. 5. Mortality Due to overexpression of Hsp70, However, Also Increases under Mild Natural Stress Regimes, Buttressing the Ecological Relevance of a Hypothesized Evolutionary Trade-Off of the Benefits and Adverse Consequences of Hsp70 Expression.