Bacteriophage Mu is one of the most efficient transposons known, capable of moving a hundred viral copies to new positions in the bacterial chromosome in an hour. Mu also forms stable lysogens. In bacteria lysogenic for the defective protein fusion-forming phage MudII1681, which can transpose and replicate but does not encode genes for DNA packaging and cell lysis, the frequency of transposition changes as colonies age. To find host genes that alter the spontaneous Mu transposition frequency, we used a genetic screen with mini-MudlacZ fusion formation as an assay. H-NS (also called H1a and B1) is an abundant nonspecific DNA-binding protein localized to the bacterial chromosome. H-NS has an unusual structure of interspersed patches of acidic and basic residues reminiscent of eukaryotic HMG proteins. Mutations in hns caused an increase in Mu-specific transcription and a dramatic increase in MudII1681 transposition rates when cells were put under certain growth conditions. Purified H-NS stabilized Mu repressor-DNA complexes in vitro, suggesting that H-NS contributes to the organization of transcriptionally inactive DNA in vivo.