Variability in the heat shock response has been studied in D. subobscura, both in individuals from different strains at the same moment of development and in individuals belonging to the same strain at different moments of prepupal development. The heat shock response was independent of the genic background of the individuals, whereas a variability depending on the moment of development has been obtained. In relation to this variability two kinds of heat shock puffs have been found: those induced independently of the moment of development and those induced with a response dependent on it. In both cases a differential genic response that is dependent on the temperature of treatment has been detected.