Abstract 18 different promoter elements, encompassing a 71-fold range of activity, were isolated from the chromosome of Lactococcus lactis (Ll) MG1363 and from an uncharacterised small isometric bacteriophage of Ll. The Vibrio fischeri (Vf) luciferase-encoding gene ( lux) was used as a reporter in Ll, so that the promoters could be identified strictly on the basis of their activity in the homologous host. Sequence and primer extension analysis of six of the promoters has provided a new consensus sequence for the −35 and −10 hexanucleotide motifs present upstream from lactococcal transcription start points. When the nucleotide sequence of the most active promoter ( P15) was compared with that of the highly expressed Ll usp45 gene, a novel 8-bp region of homology was identified which corresponded to the newly derived consensus −35 sequence element; this element may therefore be of general importance in Ll gene expression. The isolation of these promoters has also enabled us to investigate the characteristics of the Vf Lux activity in Ll under different physiological conditions using promoters of different strengths. Lux activity in Ll is critically dependent upon the phase of cell growth. Luminescence falls sharply in stationary phase, possibly due to a lack of FMNH 2. In contrast to the kinetics of Lux function in Escherichia coli (Ec), Lux activity in Ll declines rapidly after addition of the substrate; the rate of decay is dependent both on the growth phase and on the strength of the promoter. It is apparent that the previously reported thermal instability of Lux is in fact a function of the host organism in which Lux is expressed. Whereas Lux activity is inhibited at 37°C in Ec no such inhibition was observed in Ll at this temperature.