The effect of temperature on expression of fimA, the gene coding for the phase-variable type 1 fimbrial subunit protein of Escherichia coli K-12 was investigated. In a genetic background in which the orientation of the DNA fragment carrying the fimA promoter was determined by the activity of the 'orientationally unbiased' FimB recombinase, fimA transcription was consistently higher at 30 degrees C than at 37 degrees C. This apparent increase in fimA expression was found to be due to the fact that the fimA site-specific recombination system had become directionally biased at the lower temperature such that the bacterial population contained more cells in the ON phase than at 37 degrees C. When expression of fimA was studied in a strain genetically incapable of switching the fimA promoter to the OFF phase (i.e. all cells in the culture were phase-locked ON), fimA transcription was found to be higher at 37 degrees C than at 30 degrees C. Thus, transcription from the fimA promoter was subject to temperature control and the site-specific recombination system determining the orientation of the promoter DNA fragment was temperature-modulated. Furthermore, it was found that the thermosensitive fimA promoter was subject to transcriptional silencing by the HNS nucleoid protein, in a manner analogous to that described for other thermoregulated adhesins.