The 67B locus contains a cluster of heat-shock genes, four of which (hsp22, hsp23, hsp26, hsp28) are ecdysterone-inducible. Experiments on the transcription of these genes in salivary glands of pupating larvae showed that hsp23 and hsp28 mRNAs were most abundant in the larvae while hsp26 and hsp22 had intermediate or low level of accumulation, respectively. The second series of experiments have demonstrated that mutations of ecs gene or its deletion reduce either the transcription rate or stability of small hsp mRNAs. The resulting level of hsp23 and hsp28 mRNA accumulation was 70% and 40%, respectively, for the t143 mutant and 20% for the t10 mutant, as compared with the control. Mutations t435 and t324, as well as complete deletion of the ecs gene produced a greater damaging effect: the abundance of hsp23 mRNAs in the salivary glands of the appropriate larvae was reduced no less than by a factor of 10, 40 and 75, respectively. Our data show that ecs gene performs its regulatory function at the transcriptional level.