Regulation of the genes required for bioluminescence in the marine bacterium Vibrio fischeri (the lux regulon) is a complex process requiring coordination of several systems. The primary level of regulation is mediated by a positive regulatory protein, LuxR, and a small diffusible molecule, N-(3-oxo-hexanoyl)-homoserine lactone, termed autoinducer. Transcription of the luxR gene, which encodes the regulatory protein, is positively regulated by the cyclic AMP-CAP system. The lux regulon of V. fischeri consists of two divergently transcribed operons designated operonL and operonR. Transcription of the rightward operon (operonR; luxICDABE), consisting of the genes required for autoinducer synthesis (luxI) and light production (luxCDABE), is activated by LuxR in an autoinducer-dependent fashion. The leftward operon (operonL) consists of a single known gene, luxR. The LuxR protein has also been shown to decrease transcription of operonL through an autoinducer-dependent mechanism, thereby negatively regulating its own synthesis. In this paper we demonstrate that the autoinducer-dependent repression of operonL transcription requires not only LuxR but also DNA sequences within operonR which occur upstream of the promoter for operonL. In the absence of these DNA sequences, the LuxR protein causes an autoinducer-dependent activation of transcription of operonL. The lux operator, located in the control region between the two operons, was required for both the positive and negative autoinducer-dependent responses. By titration of high levels of LuxR supplied in trans with synthetic autoinducer, we found that low levels of autoinducer could elicit a positive response even in the presence of the negative-acting DNA sequences, while higher levels of autoinducer resulted in a negative response. Without these DNA sequences in operonR, LuxR and autoinducer stimulated transcription regardless of the level of autoinducer. These results suggest that a switch between stimulation and repression of operonL transcription is mediated by the levels of the LuxR-autoinducer complex, which in these experiments reflects the level of autoinducer in the growth medium.