In the psychrotrophic bacterium Pseudomonas fluorescens, some genes are thermoregulated: they are maximally expressed at a particular temperature within the broad range of temperatures that allow growth of this bacterium. To study this regulation, random transcriptional insertion fusions were obtained by means of mini-Tn5lacZ1 or mini-Tn5luxAB transposition. One fusion was studied in which beta-galactosidase production was maximal at a low-growth temperature. The mutated gene (that we call xsf) was highly homologous to xseA from Escherichia coli (and from other bacteria) which encodes the large subunit of exonuclease VII. Genetic tools were constructed in order to analyse and manipulate this fusion: a plasmid derived from R68.45 was used for chromosome transfer and a replacement vector was constructed to allow in situ marker exchange of the mini-Tn5lacZ1 by an Hg(r) interposon. This vector was used to make double mutants and hence to study the effect of the insertion in xsf on the expression of other fusions. Six genes were thereby identified with a decreased expression in an xsf- background and with different characteristics of thermoregulation.