Adult flies were collected from Suwon population and the third chromosomes were extracted from each of 112 males using the Ser/Sb balancer stocks. Then the homozygote viabilities of the third chromosome were estimated under the normal (25℃) and the heatshock(37℃) conditions. A degree of response(name as Responsability) was measured by the ration of viabilities under the heat shock and the normal condition. Furthermore, the puffing patterns of the strains which have shown high and low responsability were analyzed with heat-induced puffs on the polytene chromosome in order to observe relationship between responsability and the expression of Hsp genes. The following results were obtained. 1)The lethal frequency on the third chromosome of homozygous strains was 0.277 + 0.042. 2) The average viabilities of non-lethal homozygous strains were estimated 0.800 + 0.026 and 0.726 + 0.026 at normal and heat shock condition, respectively. 3) The average responsability was estimated 0.938 + 0.032 The wide distribution of responsabilities represented that there were many of variants for heat-shock responses in the third chromosome of the natural population. 4) Comparing with the puffing patterns on the polytene chromosome, it was observed that the duration of the puffs of 87A and 87C, which were known to code Hsp70, was longer in the strains of high responsability than in those of low one.