Six mutations, which lead to an increase in malT expression, were mapped by sequencing techniques. All of them had one or other of two base changes. Determination of the transcription start point by reverse transcriptase mapping localised the two base changes with respect to the elements that control malT expression. One of the base changes ( malTp1 ) is located in the Pribnow box of the promoter, and presumably results in an increase in the rate of transcription initiation. The other ( malTp7 ) is located in the Shine and Dalgarno sequence, which precedes the malT cistron. It probably created a more favourable ribosome binding site on malT mRNA. A correlate of these observations is that the promoter and the ribosome binding site are both inefficient in a wild-type malT gene. A malTp1 malTp7 double mutant was constructed, which produced equivalent to 30 times more MalT protein than the wild-type strain.