Heat shock has a dramatic effect on the organization of the cytoplasm, causing the intermediate filament cytoskeleton to aggregate of the nucleus. This has previously been shown in cultured Drosophila and mammalian cells. In this paper we analyze the heat lability of the intermediate filament cytoskeleton in early Drosophila embryos by indirect immunofluorescence. At all stages of embryogenesis tested, the intermediatefilamentcytoskeleton, which is maternally provided, is severely disturbed by 30 min heat shock at 37°C. After the nuclei have migrated to the subcortical cytoplasm, it collapses around them. Nuclei in all heat-shocked embryos are considerably enlarged and become displaced. Embryos before cellular blastoderm stage, in which heat shock protein synthesis is not inducible, are irreversibly arrested in development by heat shock. Embryos at or after cellular blastoderm, which do synthesize heat shock proteins in response to stress, are also immediately arrested in development but continue development when returned to 25°C. We discuss the possibility that cytoplasmic events such as the intermediate filament cytoskeleton rearrangement may be involved in heat shock-mediated phenocopy induction.