The ampC gene of Escherichia coli K-12 codes for a beta-lactamase which can hydrolyse the beta-lactam ring of ampicillin. Ampicillin resistance is strictly related to ampC gene copy number thus we have been able to isolate ampicillin-resistant mutants carrying multiple ampC repeats. We have isolated on a plasmid a segment of chromosomal DNA carrying multiple ampC repeats, and compared the nucleotide sequence of the region joining repeat units to the sequence of the DNA segments that fused to create the joint. The fusion had occurred within a 12-base pair (bp) sequence of perfect homology. We suggest that recombination between randomly occurring short homologies (12-13-bp long), could be a general mechanism to generate tandem duplications in the size range of 10 kilobases (kb).