The heat-shock consensus element (HSE), CTNGAANNTTCNAG, is found in multiple copies upstream of all heat-shock genes. Here, the sequence requirements for heat-shock induction are tested by Drosophila germline transformation with an hsp70-lacZ gene fused to a pair of synthetic HSEs. Certain single-base substitutions in either HSE cause a dramatic reduction (forty-fold) in expression. Surprisingly, variations in sequences immediately flanking the HSEs also reduced levels of induction. One such variant that contains two perfect 14-base pair HSEs, which are correctly spaced relative to each other and the TATA box, retained only 7% of wild type-induced expression. These and additional analyses indicate that the heat-shock regulatory element includes sequences beyond the 14-base pair HSE and may be better described as a dimer of a 10-base pair sequence, NTTCNNGAAN.