Abstract: The present invention relates to a process for preparing a strain of Bacillus, in the chromosome of which a specific gene has been amplified, in which process: (a) at least one plasmid integration vector bearing the said gene is integrated in the Bacillus chromosome so as to create in the chromosome: at least one DNA sequence, known as an amplifiable unit, which contains at least the said gene and its expression elements and, at each end, two sequences which are identical in the direct sense; the amplifiable unit which codes, furthermore, for a selectable gene; (b) the strains of Bacillus obtained are then selected by culture on a selection medium corresponding to the selectable gene, and the strains are withdrawn which have the phenotype corresponding to the presence of an increased number of copies of the said gene relative to the bacterial population before selection.