SummaryThe Drosophila melanogasterecd1 mutation causes a severe temperature-sensitive deficiency in the titre of the steroid hormone eedysone. This mutation was used to investigate the role of eedysone in both the transcription of the genes mapped at the 3C11–12 intermoult puff region and the puff formation. Thoroughly synchronized ecd1 larvae were shifted to the non-permissive temperature at various times of the development; after 24 or 48 h, the levels of the transcripts derived from Sgs-4, Pig-1 and ng-1, the three genes located at the 3C11–12 polytene bands, were determined. The results showed that the levels of the transcripts encoded by Pig-1 and ng-1 are unaffected by the drop in the ecdysone titre occurring in non-permissive conditions whereas the amount of Sgs-4 mRNA is greatly reduced. These data clearly indicate that transcription of the three genes mapped within the puff region is affected differently by the hormone. Furthermore, ecd1 larvae cultured at the non-permissive temperature show a prominent puff at the 3C11–12 polytene bands, indicating that eedysone is not essential for puff induction and that puff size is not simply correlated with high-level Sgs-4 transcription.