Novel transposable cloning vector and method of using it for the insertion into the chromosome of a recipient Gram-negative bacterium of DNA material foreign to that bacterium. The vector comprises first and second plasmids. The first plasmid carries a transposon (A), a part of whose DNA including the sequence which specifies its transposition function has been removed. The second plasmid carries a fragment of DNA from a transposon (A) including the sequence which specifies its transposition function. The DNA material to be inserted into the chromosome of the recipient bacterium is included in the DNA of transposon (A) carried by the first plasmid. Any suitable transposon may be used as transposon (A) but transposon Tn7 is preferred.