An artificial promoter library for a selected organism or group of organisms is constructed as a mixture of double stranded DNA fragments, the sense strands of which comprise at least two consensus sequences of efficient promoters from said organism or group of organisms, or parts thereof comprising at least half of each, and surrounding or intermediate nucleotide sequences (spacers) of variable length in which at least 7 nucleotides are selected randomly among the nucleobases A, T, C and G. The sense strands of the double stranded DNA fragments may also include a regulatory DNA sequence imparting a specific regulatory feature, such as activation by a change in the growth conditions, to the promoters of the library. Further, they may have a sequence comprising one or more recognition sites for restriction endonucleases added to one of or both their ends. The selected organism or group or organisms may be selected from prokaryotes and from eukaryotes; and in prokaryotes the consensus sequences to be retained most often will comprise the -35 signal (-35 to -30): TTGACA and the -10 signal (-12 to -7): TATAAT or parts of both comprising at least 3 conserved nucleotides of each, while in eukaryotes said consensus sequences should comprise a TATA box and at least one upstream activation sequence (UAS). Such artificial promoter libraries can be used i.a. for optimizing the expression of specific genes in various selected organisms.